drm/mediatek: Add mbox_free_channel in mtk_drm_crtc_destroy

Add mbox_free_channel in mtk_drm_crtc_destroy.

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