drm/i915: fix use-after-free in page_flip_completed()

page_flip_completed() dereferences 'work' variable after executing
queue_work(). This is not safe as the 'work' item might be already freed
by queued work:

    BUG: KASAN: use-after-free in page_flip_completed+0x3ff/0x490 at addr ffff8803dc010f90
    Call Trace:
     __asan_report_load8_noabort+0x59/0x80
     page_flip_completed+0x3ff/0x490
     intel_finish_page_flip_mmio+0xe3/0x130
     intel_pipe_handle_vblank+0x2d/0x40
     gen8_irq_handler+0x4a7/0xed0
     __handle_irq_event_percpu+0xf6/0x860
     handle_irq_event_percpu+0x6b/0x160
     handle_irq_event+0xc7/0x1b0
     handle_edge_irq+0x1f4/0xa50
     handle_irq+0x41/0x70
     do_IRQ+0x9a/0x200
     common_interrupt+0x89/0x89

    Freed:
     kfree+0x113/0x4d0
     intel_unpin_work_fn+0x29a/0x3b0
     process_one_work+0x79e/0x1b70
     worker_thread+0x611/0x1460
     kthread+0x241/0x3a0
     ret_from_fork+0x27/0x40

Move queue_work() after	trace_i915_flip_complete() to fix this.

Fixes: e5510fac ("drm/i915: add tracepoints for flip requests & completions")
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: <stable@vger.kernel.org> # v2.6.36+
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170126143211.24013-1-aryabinin@virtuozzo.com

drivers/gpu/drm/i915/intel_display.c

@@ -4253,10 +4253,10 @@ static void page_flip_completed(struct intel_crtc *intel_crtc)
 	drm_crtc_vblank_put(&intel_crtc->base);
 
 	wake_up_all(&dev_priv->pending_flip_queue);
-	queue_work(dev_priv->wq, &work->unpin_work);
-
 	trace_i915_flip_complete(intel_crtc->plane,
 				 work->pending_flip_obj);
+
+	queue_work(dev_priv->wq, &work->unpin_work);
 }
 
 static int intel_crtc_wait_for_pending_flips(struct drm_crtc *crtc)