drm/mediatek: Support DRM plane alpha in Mixer

Set the plane alpha according to DRM plane property.

Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Fixes: 119f5173 ("drm/mediatek: Add DRM Driver for Mediatek SoC MT8173.")
Signed-off-by: Hsiao Chien Sung <shawn.sung@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240620-igt-v3-14-a9d62d2e2c7e@mediatek.com/


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