According to the comment in drm_atomic_helper_async_commit(),
we should make sure FBs have been swapped, so that cleanups in the
new_state performs a cleanup in the old FB.
So we should move swapping FBs after calling mtk_plane_update_new_state(),
to avoid using the old FB which could be freed.
Fixes: 1a64a7aff8da ("drm/mediatek: Fix cursor plane no update")
Signed-off-by: Jason-JH.Lin <jason-jh.lin@xxxxxxxxxxxx>