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>

drivers/gpu/drm/mediatek/mtk_ethdr.c

@@ -170,8 +170,10 @@ void mtk_ethdr_layer_config(struct device *dev, unsigned int idx,
 		return;
 	}
 
-	if (state->base.fb && state->base.fb->format->has_alpha)
-		alpha_con = MIXER_ALPHA_AEN | MIXER_ALPHA;
+	if (state->base.fb) {
+		alpha_con |= MIXER_ALPHA_AEN;
+		alpha_con |= state->base.alpha & MIXER_ALPHA;
+	}
 
 	if (state->base.fb && !state->base.fb->format->has_alpha) {
 		/*