drm/sun4i: Add TODO comment about supporting scaling with the backend

The backend allows integer-only scaling but can handle alpha components,
unlike the frontend. It could be useful to add support for this
eventually, so add a short TODO comment describing the situation.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181123092515.2511-4-paul.kocialkowski@bootlin.com

drivers/gpu/drm/sun4i/sun4i_backend.c

@@ -426,6 +426,11 @@ static bool sun4i_backend_plane_uses_frontend(struct drm_plane_state *state)
 	if (IS_ERR(backend->frontend))
 		return false;
 
+	/*
+	 * TODO: The backend alone allows 2x and 4x integer scaling, including
+	 * support for an alpha component (which the frontend doesn't support).
+	 * Use the backend directly instead of the frontend in this case.
+	 */
 	return sun4i_backend_plane_uses_scaler(state);
 }