drm/sun4i: backend: Set a default zpos in our reset hook

The plane state zpos value will be set only if there's an existing state
attached to the plane when creating the property.

However, this is not the case during the probe, and we therefore need to
put our default value in our reset hook.
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b6a183234f0ad5a9a58c780c9cabbe29cbf40888.1516617243.git-series.maxime.ripard@free-electrons.com

drivers/gpu/drm/sun4i/sun4i_layer.c

@@ -28,6 +28,7 @@ struct sun4i_plane_desc {
 
 static void sun4i_backend_layer_reset(struct drm_plane *plane)
 {
+	struct sun4i_layer *layer = plane_to_sun4i_layer(plane);
 	struct sun4i_layer_state *state;
 
 	if (plane->state) {
@@ -43,6 +44,7 @@ static void sun4i_backend_layer_reset(struct drm_plane *plane)
 	if (state) {
 		plane->state = &state->state;
 		plane->state->plane = plane;
+		plane->state->zpos = layer->id;
 	}
 }