drm/nouveau: properly handle pushbuffer check failures

drm/nouveau: properly handle pushbuffer check failures

When "buffer in list" check does not pass, don't free validation lists - they were
not initialized yet.

Fixes this oops:

[drm] nouveau 0000:02:00.0: push 105 buffer not in list
BUG: unable to handle kernel NULL pointer dereference at 000000000000057c
IP: [<ffffffff81236aa4>] do_raw_spin_lock+0x14/0x13c
PGD 1ac6cb067 PUD 1aaa52067 PMD 0
CPU 0
Modules linked in: nouveau ttm drm_kms_helper snd_hda_codec_realtek snd_hda_intel snd_hda_codec

Pid: 6265, comm: OilRush_x86 Not tainted 2.6.38-rc6-nv+ #632 System manufacturer System Product Name/P6T SE
RIP: 0010:[<ffffffff81236aa4>]  [<ffffffff81236aa4>] do_raw_spin_lock+0x14/0x13c
(...)
Process OilRush_x86 (pid: 6265, threadinfo ffff8801a6aee000, task ffff8801a26c0000)
 0000000000000000 ffff8801ac74c618 0000000000000000 0000000000000578
 0000000000000000 ffff8801ac74c618 0000000000000000 ffff8801bd9d0000
 [<ffffffff81417f78>] _raw_spin_lock+0x1e/0x22
 [<ffffffffa00a2746>] nouveau_bo_fence+0x2e/0x60 [nouveau]
 [<ffffffffa00a540b>] validate_fini_list+0x35/0xeb [nouveau]
 [<ffffffffa00a54d3>] validate_fini+0x12/0x31 [nouveau]
 [<ffffffffa00a6386>] nouveau_gem_ioctl_pushbuf+0xe94/0xf6b [nouveau]
 [<ffffffff8141ac56>] ? sub_preempt_count+0x9e/0xb2
 [<ffffffff81417e94>] ? _raw_spin_unlock_irqrestore+0x30/0x4d
 [<ffffffff8105dea2>] ? __wake_up+0x3f/0x48
 [<ffffffff812aebb4>] drm_ioctl+0x289/0x361
 [<ffffffff8141ac56>] ? sub_preempt_count+0x9e/0xb2
 [<ffffffffa00a54f2>] ? nouveau_gem_ioctl_pushbuf+0x0/0xf6b [nouveau]
 [<ffffffff8141ac56>] ? sub_preempt_count+0x9e/0xb2
 [<ffffffffa010caa2>] nouveau_compat_ioctl+0x16/0x1c [nouveau]
 [<ffffffff81142c0d>] compat_sys_ioctl+0x1c8/0x12d7
 [<ffffffff814179ca>] ? trace_hardirqs_off_thunk+0x3a/0x6c
 [<ffffffff81058099>] sysenter_dispatch+0x7/0x30
 [<ffffffff8141798e>] ? trace_hardirqs_on_thunk+0x3a/0x3c
RIP  [<ffffffff81236aa4>] do_raw_spin_lock+0x14/0x13c
 RSP <ffff8801a6aefb88>
---[ end trace 0014d5d93e6147e1 ]---

Additionally, don't call validate_fini twice in case of validation failure.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Maarten Maathuis <madman2003@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>