drm/mediatek: Fix bit depth overwritten for mtk_ovl_set bit_depth()

Refine the value and mask define of bit depth for mtk_ovl_set bit_depth().
Use cmdq_pkt_write_mask() instead of cmdq_pkt_write() to avoid bit depth
settings being overwritten.

Fixes: fb36c502 ("drm/mediatek: Add support for AR30 and BA30 overlays")
Signed-off-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240624095726.18818-1-jason-jh.lin@mediatek.com/


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