drm/sun4i: backend: Make zpos configurable

Now that we have everything in place, we can make zpos configurable now.
Change the zpos property from an immutable one to a regular.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0ab187956855db86972d936e6751181649e0d035.1518802627.git-series.maxime.ripard@bootlin.com

drivers/gpu/drm/sun4i/sun4i_layer.c

@@ -163,6 +163,9 @@ static struct sun4i_layer *sun4i_layer_init_one(struct drm_device *drm,
 			     &sun4i_backend_layer_helper_funcs);
 	layer->backend = backend;
 
+	drm_plane_create_zpos_property(&layer->plane, 0, 0,
+				       SUN4I_BACKEND_NUM_LAYERS - 1);
+
 	return layer;
 }
 
@@ -190,8 +193,6 @@ struct drm_plane **sun4i_layers_init(struct drm_device *drm,
 			return ERR_CAST(layer);
 		};
 
-		drm_plane_create_zpos_immutable_property(&layer->plane, i);
-
 		layer->id = i;
 		planes[i] = &layer->plane;
 	};