drm/i915: Drain any pending flips on the fb prior to unpinning

If we have queued a page flip on the current fb and then request a mode
change, wait until the page flip completes before performing the new
request.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 37a44c8..ce8ff8f 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1024,6 +1024,9 @@
 			 uint32_t seqno,
 			 bool interruptible,
 			 struct intel_ring_buffer *ring);
+int i915_gem_wait_for_pending_flip(struct drm_device *dev,
+				   struct drm_gem_object **object_list,
+				   int count);
 int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf);
 int i915_gem_object_set_to_gtt_domain(struct drm_gem_object *obj,
 				      int write);