drm: rcar-du: Set primary plane zpos immutably at initializing

According to drm_plane_create_zpos_property() function documentation,
all planes zpos range should be set if zpos property is supported.
However, the rcar-du driver didn't set primary plane zpos range. Since
the primary plane's zpos is fixed, set it immutably.
Reported-by: Yoshihito Ogawa <yoshihito.ogawa.kc@renesas.com>
Reported-by: Koji Matsuoka <koji.matsuoka.xm@renesas.com>
Signed-off-by: Tomohito Esaki <etom@igel.co.jp>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
[Turn continue into if ... else ...]
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

drivers/gpu/drm/rcar-du/rcar_du_plane.c

@@ -785,14 +785,16 @@ int rcar_du_planes_init(struct rcar_du_group *rgrp)
 
 		drm_plane_create_alpha_property(&plane->plane);
 
-		if (type == DRM_PLANE_TYPE_PRIMARY)
-			continue;
-
+		if (type == DRM_PLANE_TYPE_PRIMARY) {
+			drm_plane_create_zpos_immutable_property(&plane->plane,
+								 0);
+		} else {
 			drm_object_attach_property(&plane->plane.base,
 						   rcdu->props.colorkey,
 						   RCAR_DU_COLORKEY_NONE);
 			drm_plane_create_zpos_property(&plane->plane, 1, 1, 7);
 		}
+	}
 
 	return 0;
 }

drivers/gpu/drm/rcar-du/rcar_du_vsp.c

@@ -392,13 +392,15 @@ int rcar_du_vsp_init(struct rcar_du_vsp *vsp, struct device_node *np,
 		drm_plane_helper_add(&plane->plane,
 				     &rcar_du_vsp_plane_helper_funcs);
 
-		if (type == DRM_PLANE_TYPE_PRIMARY)
-			continue;
-
+		if (type == DRM_PLANE_TYPE_PRIMARY) {
+			drm_plane_create_zpos_immutable_property(&plane->plane,
+								 0);
+		} else {
 			drm_plane_create_alpha_property(&plane->plane);
 			drm_plane_create_zpos_property(&plane->plane, 1, 1,
 						       vsp->num_planes - 1);
 		}
+	}
 
 	return 0;
 }