drm/i915: check for kernel_context

drm/i915: check for kernel_context

Explosions during early driver init on the error path. Make sure we fail
gracefully.

[ 9547.672258] BUG: kernel NULL pointer dereference, address: 000000000000007c
[ 9547.672288] #PF: supervisor read access in kernel mode
[ 9547.672292] #PF: error_code(0x0000) - not-present page
[ 9547.672296] PGD 8000000846b41067 P4D 8000000846b41067 PUD 797034067 PMD 0
[ 9547.672303] Oops: 0000 [#1] SMP PTI
[ 9547.672307] CPU: 1 PID: 25634 Comm: i915_selftest Tainted: G     U            5.3.0-rc8+ #73
[ 9547.672313] Hardware name:  /NUC6i7KYB, BIOS KYSKLi70.86A.0050.2017.0831.1924 08/31/2017
[ 9547.672395] RIP: 0010:intel_context_unpin+0x9/0x100 [i915]
[ 9547.672400] Code: 6b 60 00 e9 17 ff ff ff bd fc ff ff ff e9 7c ff ff ff 66 66 2e 0f 1f 84 00 00 00 00
 00 0f 1f 40 00 0f 1f 44 00 00 41 54 55 53 <8b> 47 7c 83 f8 01 74 26 8d 48 ff f0 0f b1 4f 7c 48 8d 57 7c
 75 05
[ 9547.672413] RSP: 0018:ffffae8ac24ff878 EFLAGS: 00010246
[ 9547.672417] RAX: ffff944a1b7842d0 RBX: ffff944a1b784000 RCX: ffff944a12dd6fa8
[ 9547.672422] RDX: ffff944a1b7842c0 RSI: ffff944a12dd5328 RDI: 0000000000000000
[ 9547.672428] RBP: 0000000000000000 R08: ffff944a11e5d840 R09: 0000000000000000
[ 9547.672433] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
[ 9547.672438] R13: ffffffffc11aaf00 R14: 00000000ffffffe4 R15: ffff944a0e29bf38
[ 9547.672443] FS:  00007fc259b88ac0(0000) GS:ffff944a1f880000(0000) knlGS:0000000000000000
[ 9547.672449] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 9547.672454] CR2: 000000000000007c CR3: 0000000853346003 CR4: 00000000003606e0
[ 9547.672459] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 9547.672464] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 9547.672469] Call Trace:
[ 9547.672518]  intel_engine_cleanup_common+0xe3/0x270 [i915]
[ 9547.672567]  execlists_destroy+0xe/0x30 [i915]
[ 9547.672669]  intel_engines_init+0x94/0xf0 [i915]
[ 9547.672749]  i915_gem_init+0x191/0x950 [i915]

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927173409.31175-2-matthew.auld@intel.com