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: fb36c5020c9c ("drm/mediatek: Add support for AR30 and BA30 overlays")
Signed-off-by: Jason-JH.Lin <jason-jh.lin@xxxxxxxxxxxx>