In the event that we fail to dispatch the execbuffer, for example if
there is insufficient space on the ring, we were leaving the objects in
an inconsistent state. Notably they were marked as being in the GPU
write domain, but were not added to the ring or any list. This would
lead to inevitable oops:

[ 1010.522940] [drm:i915_gem_do_execbuffer] *ERROR* dispatch failed -16
[ 1010.523055] BUG: unable to handle kernel NULL pointer dereference at
0000000000000088
[ 1010.523097] IP: [<ffffffff8122d006>] i915_gem_flush_ring+0x26/0x140
[ 1010.523120] PGD 14cf2f067 PUD 14ce04067 PMD 0
[ 1010.523140] Oops: 0000 [#1] SMP
[ 1010.523154] last sysfs file: /sys/devices/virtual/vc/vcsa2/uevent
[ 1010.523173] CPU 0
[ 1010.523183] Pid: 716, comm: X Not tainted 2.6.36+ #34 LosLunas
CRB/SandyBridge Platform
[ 1010.523206] RIP: 0010:[<ffffffff8122d006>]  [<ffffffff8122d006>]
i915_gem_flush_ring+0x26/0x140
[ 1010.523233] RSP: 0018:ffff88014bf97cd8  EFLAGS: 00010296
[ 1010.523249] RAX: ffff88014e2d1808 RBX: 0000000000000000 RCX: 0000000000000000
[ 1010.523270] RDX: 0000000000000002 RSI: 0000000000000000 RDI: 0000000000000000
[ 1010.523290] RBP: ffff88014e2d1000 R08: 0000000000000002 R09: 00000000400c645f
[ 1010.523311] R10: 0000000000000001 R11: 0000000000000246 R12: 0000000000000002
[ 1010.523331] R13: ffff88014e29a000 R14: 00000000000000c8 R15: ffffffff8162eb28
[ 1010.523352] FS:  00007fc62379d700(0000) GS:ffff88001fc00000(0000) knlGS:0000000000000000
[ 1010.523375] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1010.523392] CR2: 0000000000000088 CR3: 000000014bf87000 CR4: 00000000000406f0
[ 1010.523412] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 1010.523433] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[ 1010.523454] Process X (pid: 716, threadinfo ffff88014bf96000, task ffff88014cc1ee40)
[ 1010.523475] Stack:
[ 1010.523483]  ffff88014d5199c0 0000000000000200 0000000000000000 ffff88014bcc6400
[ 1010.523509] <0> 0000000000000000 0000000000000001 ffff88014e29a000 ffff88014bcc6400
[ 1010.523537] <0> ffffffff8162eb28 ffffffff8122faa8 ffff88014e29a000 ffff88014bcc6400
[ 1010.523568] Call Trace:
[ 1010.523578]  [<ffffffff8122faa8>] ?  i915_gem_object_flush_gpu_write_domain+0x48/0x80
[ 1010.523601]  [<ffffffff8122fb8e>] ?  i915_gem_object_set_to_gtt_domain+0x2e/0xb0
[ 1010.523623]  [<ffffffff8123113b>] ?  i915_gem_set_domain_ioctl+0xdb/0x1f0
[ 1010.523644]  [<ffffffff8120a3f1>] ? drm_ioctl+0x3d1/0x460
[ 1010.523660]  [<ffffffff81231060>] ?  i915_gem_set_domain_ioctl+0x0/0x1f0
[ 1010.523682]  [<ffffffff81092618>] ?  vma_prio_tree_insert+0x28/0x120
[ 1010.523701]  [<ffffffff8109f379>] ? vma_link+0x99/0xf0
[ 1010.523717]  [<ffffffff810a111d>] ? mmap_region+0x1ed/0x4f0
[ 1010.523734]  [<ffffffff810c306f>] ? do_vfs_ioctl+0x9f/0x580
[ 1010.523750]  [<ffffffff810c3599>] ? sys_ioctl+0x49/0x80
[ 1010.523767]  [<ffffffff810022eb>] ?  system_call_fastpath+0x16/0x1b
[ 1010.523785] Code: 00 00 00 00 00 41 57 89 ce 41 56 41 55 41 54 45 89 c4 55 48 89 fd 53 48 89 d3 44 89 c2 48 89 df 4c 8d b3 c8 00 00 00 48 83 ec 18 <ff> 93 88 00 00 00 48 8b 83 c8 00 00 00 4c 8b bd 30 03 00 00 48
[ 1010.523946] RIP  [<ffffffff8122d006>] i915_gem_flush_ring+0x26/0x140
[ 1010.523966]  RSP <ffff88014bf97cd8>
[ 1010.523977] CR2: 0000000000000088
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>