If we are really unlucky and encounter an error during
i915_vm_alloc_pt_stash, we end up passing an empty pt/pd stash all the
way down into the low-level ppgtt alloc code, leading to explosions,
since it expects at least the required number of pt/pd for the va range.

[  211.981418] BUG: kernel NULL pointer dereference, address: 0000000000000000
[  211.981421] #PF: supervisor read access in kernel mode
[  211.981422] #PF: error_code(0x0000) - not-present page
[  211.981424] PGD 80000008439cb067 P4D 80000008439cb067 PUD 84a37f067 PMD 0
[  211.981427] Oops: 0000 [#1] SMP PTI
[  211.981428] CPU: 1 PID: 1301 Comm: i915_selftest Tainted: G     U    I       5.9.0-rc5+ #3
[  211.981430] Hardware name:  /NUC6i7KYB, BIOS KYSKLi70.86A.0050.2017.0831.1924 08/31/2017
[  211.981521] RIP: 0010:__gen8_ppgtt_alloc+0x1ed/0x3c0 [i915]
[  211.981523] Code: c1 48 c7 c7 5d 5d fe c0 65 ff 0d ee 1d 03 3f e8 d9 91 1f e2 8b 55 c4 31 c0 48 8b 75 b8 85 d2 0f 95 c0 48 8b 1c c6 48 89 45 98 <48> 8b 03 48 8b 90 58 02 00 00 48 85 d2 0f 84 07 ea 15 00 48 81 fa
[  211.981526] RSP: 0018:ffffba2cc0eb3970 EFLAGS: 00010202
[  211.981527] RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000004
[  211.981529] RDX: 0000000000000002 RSI: ffff9be998bdb8c0 RDI: ffff9be99c844300
[  211.981530] RBP: ffffba2cc0eb39d8 R08: 0000000000000640 R09: ffff9be97cdfd000
[  211.981531] R10: ffff9be97cdfd614 R11: 0000000000000000 R12: 0000000000000000
[  211.981532] R13: ffff9be98607ba20 R14: ffff9be995a0b400 R15: ffffba2cc0eb39e8
[  211.981534] FS:  00007f0f10b31000(0000) GS:ffff9be99fc40000(0000) knlGS:0000000000000000
[  211.981536] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  211.981538] CR2: 0000000000000000 CR3: 000000084d74e006 CR4: 00000000003706e0
[  211.981539] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  211.981541] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  211.981542] Call Trace:
[  211.981609]  gen8_ppgtt_alloc+0x79/0x90 [i915]
[  211.981678]  ppgtt_bind_vma+0x36/0x80 [i915]
[  211.981756]  __vma_bind+0x39/0x40 [i915]
[  211.981818]  fence_work+0x21/0x98 [i915]
[  211.981879]  fence_notify+0x8d/0x128 [i915]
[  211.981939]  __i915_sw_fence_complete+0x62/0x240 [i915]
[  211.982018]  i915_vma_pin_ww+0x1ee/0x9c0 [i915]

Fixes: cd0452aa ("drm/i915: Preallocate stashes for vma page-directories")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
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/20200921160844.73186-1-matthew.auld@intel.com
(cherry picked from commit 1604cb2a)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>