drm/msm: fix fb references in async update

Async update callbacks are expected to set the old_fb in the new_state
so prepare/cleanup framebuffers are balanced.

Cc: <stable@vger.kernel.org> # v4.14+
Fixes: 224a4c97 ("drm/msm: update cursors asynchronously through atomic")
Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Acked-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190603165610.24614-4-helen.koike@collabora.com

drivers/gpu/drm/msm/disp/mdp5/mdp5_plane.c

@@ -502,6 +502,8 @@ static int mdp5_plane_atomic_async_check(struct drm_plane *plane,
 static void mdp5_plane_atomic_async_update(struct drm_plane *plane,
 					   struct drm_plane_state *new_state)
 {
+	struct drm_framebuffer *old_fb = plane->state->fb;
+
 	plane->state->src_x = new_state->src_x;
 	plane->state->src_y = new_state->src_y;
 	plane->state->crtc_x = new_state->crtc_x;
@@ -524,6 +526,8 @@ static void mdp5_plane_atomic_async_update(struct drm_plane *plane,
 
 	*to_mdp5_plane_state(plane->state) =
 		*to_mdp5_plane_state(new_state);
+
+	new_state->fb = old_fb;
 }
 
 static const struct drm_plane_helper_funcs mdp5_plane_helper_funcs = {