drm/mediatek: Add mtk_dither_set_common() function

Current implementation of mtk_dither_set() cast dev data to
struct mtk_ddp_comp_dev. But other devices with different dev data
would also call this function.

Separate necessary parameters out so other device components (dither,
gamma) can call this function.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>