drm/msm: use drm_atomic_set_fence_for_plane() to set the fence

drm_atomic_set_fence_for_plane() is smart and won't overwrite
plane_state->fence if the user already set an explicit fence there.

Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1478513013-3221-3-git-send-email-gustavo@padovan.org

drivers/gpu/drm/msm/msm_atomic.c

@@ -217,8 +217,9 @@ int msm_atomic_commit(struct drm_device *dev,
 		if ((plane->state->fb != plane_state->fb) && plane_state->fb) {
 			struct drm_gem_object *obj = msm_framebuffer_bo(plane_state->fb, 0);
 			struct msm_gem_object *msm_obj = to_msm_bo(obj);
+			struct dma_fence *fence = reservation_object_get_excl_rcu(msm_obj->resv);
 
-			plane_state->fence = reservation_object_get_excl_rcu(msm_obj->resv);
+			drm_atomic_set_fence_for_plane(plane_state, fence);
 		}
 	}