drm/mediatek: Use mailbox rx_callback instead of cmdq_task_cb

rx_callback is a standard mailbox callback mechanism and could cover the
function of proprietary cmdq_task_cb, so use the standard one instead of
the proprietary one.

Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Signed-off-by: jason-jh.lin <jason-jh.lin@mediatek.com>