drm/mediatek: add the comment about color format setting for OVL

This patch add the comment about color format setting for OVL
Signed-off-by: Stu Hsieh <stu.hsieh@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>

drivers/gpu/drm/mediatek/mtk_disp_ovl.c

@@ -157,6 +157,11 @@ static void mtk_ovl_layer_off(struct mtk_ddp_comp *comp, unsigned int idx)
 
 static unsigned int ovl_fmt_convert(struct mtk_disp_ovl *ovl, unsigned int fmt)
 {
+	/* The return value in switch "MEM_MODE_INPUT_FORMAT_XXX"
+	 * is defined in mediatek HW data sheet.
+	 * The alphabet order in XXX is no relation to data
+	 * arrangement in memory.
+	 */
 	switch (fmt) {
 	default:
 	case DRM_FORMAT_RGB565: