drm/mediatek: Support DRM plane alpha in OVL

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-v3-13-a9d62d2e2c7e@mediatek.com/Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

drivers/gpu/drm/mediatek/mtk_disp_ovl.c

@@ -458,8 +458,10 @@ void mtk_ovl_layer_config(struct device *dev, unsigned int idx,
 	}
 
 	con = ovl_fmt_convert(ovl, fmt);
-	if (state->base.fb && state->base.fb->format->has_alpha)
-		con |= OVL_CON_AEN | OVL_CON_ALPHA;
+	if (state->base.fb) {
+		con |= OVL_CON_AEN;
+		con |= state->base.alpha & OVL_CON_ALPHA;
+	}
 
 	/* CONST_BLD must be enabled for XRGB formats although the alpha channel
 	 * can be ignored, or OVL will still read the value from memory.