drm/mediatek: Add fb swap in async_update

Besides x, y position, width and height,
fb also need updating in async update.

Fixes: 920fffcc

 ("drm/mediatek: update cursors by using async atomic update")

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>