mailbox: Remove WARN_ON for async_cb.cb in cmdq_exec_done

Because mtk_drm_crtc_update_config is not using cmdq_pkt_flush_async,
it won't have pkt->async_cb.cb anymore.

So remove the WARN_ON check of pkt->async_cb.cb at cmdq_exec_done.

Fixes: 1b6b0ce2 ("mailbox: mtk-cmdq: Use mailbox rx_callback")
Signed-off-by: jason-jh.lin <jason-jh.lin@mediatek.com>
Reviewed-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>