drm/mediatek: Remove struct cmdq_client

In mailbox rx_callback, it pass struct mbox_client to callback
function, but it could not map back to mtk_drm_crtc instance
because struct cmdq_client use a pointer to struct mbox_client:

struct cmdq_client {
	struct mbox_client client;
	struct mbox_chan *chan;
};

struct mtk_drm_crtc {
	/* client instance data */
	struct cmdq_client *cmdq_client;
};

so remove struct cmdq_client and let mtk_drm_crtc instance define
mbox_client as:

struct mtk_drm_crtc {
	/* client instance data */
	struct mbox_client cl;
};

and in rx_callback function, use struct mbox_client to get
struct mtk_drm_crtc.

Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>