drm/i915/gem: Protect the obj->vma.list during iteration

drm/i915/gem: Protect the obj->vma.list during iteration

Take the obj->vma.lock to prevent modifications to the list as we
iterate, to avoid the dreaded NULL pointer.

<1>[  347.820823] BUG: kernel NULL pointer dereference, address: 0000000000000150
<1>[  347.820856] #PF: supervisor read access in kernel mode
<1>[  347.820874] #PF: error_code(0x0000) - not-present page
<6>[  347.820892] PGD 0 P4D 0
<4>[  347.820908] Oops: 0000 [#1] PREEMPT SMP NOPTI
<4>[  347.820926] CPU: 3 PID: 1303 Comm: gem_persistent_ Tainted: G     U            5.4.0-rc7-CI-CI_DRM_7352+ #1
<4>[  347.820956] Hardware name:  /NUC6CAYB, BIOS AYAPLCEL.86A.0049.2018.0508.1356 05/08/2018
<4>[  347.821132] RIP: 0010:i915_gem_object_flush_write_domain+0xd9/0x1d0 [i915]
<4>[  347.821157] Code: 0f 84 e9 00 00 00 48 8b 80 e0 fd ff ff f6 c4 40 75 11 e9 ed 00 00 00 48 8b 80 e0 fd ff ff f6 c4 40 74 26 48 8b 83 b0 00 00 00 <48> 8b b8 50 01 00 00 e8 fb 20 fb ff 48 8b 83 30 03 00 00 49 39 c4
<4>[  347.821210] RSP: 0018:ffffc90000a1f8f8 EFLAGS: 00010202
<4>[  347.821229] RAX: 0000000000000000 RBX: ffffc900008479a0 RCX: 0000000000000018
<4>[  347.821252] RDX: 0000000000000000 RSI: 000000000000000d RDI: ffff888275a090b0
<4>[  347.821274] RBP: ffff8882673c8040 R08: ffff88825991b8d0 R09: 0000000000000000
<4>[  347.821297] R10: 0000000000000000 R11: 0000000000000000 R12: ffff8882673c8280
<4>[  347.821319] R13: ffff8882673c8368 R14: 0000000000000000 R15: ffff888266a54000
<4>[  347.821343] FS:  00007f75865f4240(0000) GS:ffff888277b80000(0000) knlGS:0000000000000000
<4>[  347.821368] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
<4>[  347.821389] CR2: 0000000000000150 CR3: 000000025aee0000 CR4: 00000000003406e0
<4>[  347.821411] Call Trace:
<4>[  347.821555]  i915_gem_object_prepare_read+0xea/0x2a0 [i915]
<4>[  347.821706]  intel_engine_cmd_parser+0x5ce/0xe90 [i915]
<4>[  347.821834]  ? __i915_sw_fence_complete+0x1a0/0x250 [i915]
<4>[  347.821990]  i915_gem_do_execbuffer+0xb4c/0x2550 [i915]

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191119100929.2628356-8-chris@chris-wilson.co.uk