drm/mediatek: Support "None" blending in Mixer

Support "None" alpha blending mode on MediaTek's chips.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hsiao Chien Sung <shawn.sung@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240717-alpha-blending-v4-2-4b1c806c0749@mediatek.com/


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