drm/i915/ppgtt: Defer request freeing on reset

drm/i915/ppgtt: Defer request freeing on reset

We need to defer the free request until the object/vma is capable of
being freed - or else we have a problem  when we try to destroy the
context.

The exact same issue is described and fixed here:
commit e20780439b26ba95aeb29d3e27cd8cc32bc82a4c
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Fri Dec 6 14:11:22 2013 -0800

    drm/i915: Defer request freeing

I had this fix previously, but decided not to keep it for some reason I
can no longer remember.

gem_reset_stats is a really good test at hitting the problem.

For the inquisitive:
[  170.516392] ------------[ cut here ]------------
[  170.517227] WARNING: CPU: 1 PID: 105 at drivers/gpu/drm/drm_mm.c:578 drm_mm_takedown+0x2e/0x30 [drm]()
[  170.518064] Memory manager not clean during takedown.
[  170.518941] CPU: 1 PID: 105 Comm: kworker/1:1 Not tainted 3.13.0-rc4-BEN+ #28
[  170.519787] Hardware name: Hewlett-Packard HP EliteBook 8470p/179B, BIOS 68ICF Ver. F.02 04/27/2012
[  170.520662] Call Trace:
[  170.521517]  [<ffffffff814f0589>] dump_stack+0x4e/0x7a
[  170.522373]  [<ffffffff81049e6d>] warn_slowpath_common+0x7d/0xa0
[  170.523227]  [<ffffffff81049edc>] warn_slowpath_fmt+0x4c/0x50
[  170.524079]  [<ffffffffa06c414e>] drm_mm_takedown+0x2e/0x30 [drm]
[  170.524934]  [<ffffffffa07213f3>] gen6_ppgtt_cleanup+0x23/0x110
[i915]
[  170.525777]  [<ffffffffa07837ed>] ppgtt_release.part.5+0x24/0x29
[i915]
[  170.526603]  [<ffffffffa071aaa5>] i915_gem_context_free+0x195/0x1a0
[i915]
[  170.527423]  [<ffffffffa071189d>] i915_gem_free_request+0x9d/0xb0
[i915]
[  170.528247]  [<ffffffffa0718af9>] i915_gem_reset+0x1f9/0x3f0 [i915]
[  170.529065]  [<ffffffffa0700cce>] i915_reset+0x4e/0x180 [i915]
[  170.529870]  [<ffffffffa070829d>] i915_error_work_func+0xcd/0x120
[i915]
[  170.530666]  [<ffffffff8106c13a>] process_one_work+0x1fa/0x6d0
[  170.531453]  [<ffffffff8106c0d8>] ? process_one_work+0x198/0x6d0
[  170.532230]  [<ffffffff8106c72b>] worker_thread+0x11b/0x3a0
[  170.532996]  [<ffffffff8106c610>] ? process_one_work+0x6d0/0x6d0
[  170.533771]  [<ffffffff810743ef>] kthread+0xff/0x120
[  170.534548]  [<ffffffff810742f0>] ? insert_kthread_work+0x80/0x80
[  170.535322]  [<ffffffff814f97ac>] ret_from_fork+0x7c/0xb0
[  170.536089]  [<ffffffff810742f0>] ? insert_kthread_work+0x80/0x80
[  170.536847] ---[ end trace 3d4c12892e42d58f ]---

v2: Whitespace fix. (Chris)

Note: This is a bug that only hits the ppgtt topic branch but I've
figured that doing the request cleanup in this order is generally the
right thing to do.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Add a code comment to clarify what's actually going on since
the lifetime rules aroung ppgtt cleanup are ... fuzzy a best atm. Also
add a note about why we need this.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>