drm/meson: Add zpos immutable property to planes

Add immutable zpos property to primary and overlay planes to specify
the current fixed zpos position.

Fixes: f9a23481 ("drm/meson: Support Overlay plane for video rendering")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190429075247.7946-1-narmstrong@baylibre.com

drivers/gpu/drm/meson/meson_overlay.c

@@ -578,6 +578,9 @@ int meson_overlay_create(struct meson_drm *priv)
 
 	drm_plane_helper_add(plane, &meson_overlay_helper_funcs);
 
+	/* For now, VD Overlay plane is always on the back */
+	drm_plane_create_zpos_immutable_property(plane, 0);
+
 	priv->overlay_plane = plane;
 
 	DRM_DEBUG_DRIVER("\n");

drivers/gpu/drm/meson/meson_plane.c

@@ -371,6 +371,9 @@ int meson_plane_create(struct meson_drm *priv)
 
 	drm_plane_helper_add(plane, &meson_plane_helper_funcs);
 
+	/* For now, OSD Primary plane is always on the front */
+	drm_plane_create_zpos_immutable_property(plane, 1);
+
 	priv->primary_plane = plane;
 
 	return 0;