drm/amdgpu: Register VGA clients after init can no longer fail

drm/amdgpu: Register VGA clients after init can no longer fail

When an amdgpu device fails to init, it makes another VGA device cause
kernel splat:
kernel: amdgpu 0000:08:00.0: amdgpu: amdgpu_device_ip_init failed
kernel: amdgpu 0000:08:00.0: amdgpu: Fatal error during GPU init
kernel: amdgpu: probe of 0000:08:00.0 failed with error -110
...
kernel: amdgpu 0000:01:00.0: vgaarb: changed VGA decodes: olddecodes=io+mem,decodes=none:owns=none
kernel: BUG: kernel NULL pointer dereference, address: 0000000000000018
kernel: #PF: supervisor read access in kernel mode
kernel: #PF: error_code(0x0000) - not-present page
kernel: PGD 0 P4D 0
kernel: Oops: 0000 [#1] SMP NOPTI
kernel: CPU: 6 PID: 1080 Comm: Xorg Tainted: G        W         5.12.0-rc8+ #12
kernel: Hardware name: HP HP EliteDesk 805 G6/872B, BIOS S09 Ver. 02.02.00 12/30/2020
kernel: RIP: 0010:amdgpu_device_vga_set_decode+0x13/0x30 [amdgpu]
kernel: Code: 06 31 c0 c3 b8 ea ff ff ff 5d c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 0f 1f 44 00 00 55 48 8b 87 90 06 00 00 48 89 e5 53 89 f3 <48> 8b 40 18 40 0f b6 f6 e8 40 58 39 fd 80 fb 01 5b 5d 19 c0 83 e0
kernel: RSP: 0018:ffffae3c0246bd68 EFLAGS: 00010002
kernel: RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
kernel: RDX: ffff8dd1af5a8560 RSI: 0000000000000000 RDI: ffff8dce8c160000
kernel: RBP: ffffae3c0246bd70 R08: ffff8dd1af5985c0 R09: ffffae3c0246ba38
kernel: R10: 0000000000000001 R11: 0000000000000001 R12: 0000000000000246
kernel: R13: 0000000000000000 R14: 0000000000000003 R15: ffff8dce81490000
kernel: FS:  00007f9303d8fa40(0000) GS:ffff8dd1af580000(0000) knlGS:0000000000000000
kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
kernel: CR2: 0000000000000018 CR3: 0000000103cfa000 CR4: 0000000000350ee0
kernel: Call Trace:
kernel:  vga_arbiter_notify_clients.part.0+0x4a/0x80
kernel:  vga_get+0x17f/0x1c0
kernel:  vga_arb_write+0x121/0x6a0
kernel:  ? apparmor_file_permission+0x1c/0x20
kernel:  ? security_file_permission+0x30/0x180
kernel:  vfs_write+0xca/0x280
kernel:  ksys_write+0x67/0xe0
kernel:  __x64_sys_write+0x1a/0x20
kernel:  do_syscall_64+0x38/0x90
kernel:  entry_SYSCALL_64_after_hwframe+0x44/0xae
kernel: RIP: 0033:0x7f93041e02f7
kernel: Code: 75 05 48 83 c4 58 c3 e8 f7 33 ff ff 0f 1f 80 00 00 00 00 f3 0f 1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 51 c3 48 83 ec 28 48 89 54 24 18 48 89 74 24
kernel: RSP: 002b:00007fff60e49b28 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
kernel: RAX: ffffffffffffffda RBX: 000000000000000b RCX: 00007f93041e02f7
kernel: RDX: 000000000000000b RSI: 00007fff60e49b40 RDI: 000000000000000f
kernel: RBP: 00007fff60e49b40 R08: 00000000ffffffff R09: 00007fff60e499d0
kernel: R10: 00007f93049350b5 R11: 0000000000000246 R12: 000056111d45e808
kernel: R13: 0000000000000000 R14: 000056111d45e7f8 R15: 000056111d46c980
kernel: Modules linked in: nls_iso8859_1 snd_hda_codec_realtek snd_hda_codec_generic ledtrig_audio snd_hda_codec_hdmi snd_hda_intel snd_intel_dspcfg snd_hda_codec snd_hwdep snd_hda_core snd_pcm snd_seq input_leds snd_seq_device snd_timer snd soundcore joydev kvm_amd serio_raw k10temp mac_hid hp_wmi ccp kvm sparse_keymap wmi_bmof ucsi_acpi efi_pstore typec_ucsi rapl typec video wmi sch_fq_codel parport_pc ppdev lp parport ip_tables x_tables autofs4 btrfs blake2b_generic zstd_compress raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx libcrc32c xor raid6_pq raid1 raid0 multipath linear dm_mirror dm_region_hash dm_log hid_generic usbhid hid amdgpu drm_ttm_helper ttm iommu_v2 gpu_sched i2c_algo_bit drm_kms_helper syscopyarea sysfillrect crct10dif_pclmul sysimgblt crc32_pclmul fb_sys_fops ghash_clmulni_intel cec rc_core aesni_intel crypto_simd psmouse cryptd r8169 i2c_piix4 drm ahci xhci_pci realtek libahci xhci_pci_renesas gpio_amdpt gpio_generic
kernel: CR2: 0000000000000018
kernel: ---[ end trace 76d04313d4214c51 ]---

Commit 4192f7b57689 ("drm/amdgpu: unmap register bar on device init
failure") makes amdgpu_driver_unload_kms() skips amdgpu_device_fini(),
so the VGA clients remain registered. So when
vga_arbiter_notify_clients() iterates over registered clients, it causes
NULL pointer dereference.

Since there's no reason to register VGA clients that early, so solve
the issue by putting them after all the goto cleanups.

v2:
 - Remove redundant vga_switcheroo cleanup in failed: label.

Fixes: 4192f7b57689 ("drm/amdgpu: unmap register bar on device init failure")
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>