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 <[e-mail]>
Reported-by: Koji Matsuoka <[e-mail]>
Signed-off-by: Tomohito Esaki <[e-mail]>
Reviewed-by: Laurent Pinchart <[e-mail]>
Reviewed-by: Daniel Stone <[e-mail]>
[Turn continue into if ... else ...]
Signed-off-by: Laurent Pinchart <[e-mail]>

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

···
 
 		drm_plane_create_alpha_property(&plane->plane);
 
-		if (type == DRM_PLANE_TYPE_PRIMARY)
-			continue;
-
-		drm_object_attach_property(&plane->plane.base,
-					   rcdu->props.colorkey,
-					   RCAR_DU_COLORKEY_NONE);
-		drm_plane_create_zpos_property(&plane->plane, 1, 1, 7);
+		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

···
 		drm_plane_helper_add(&plane->plane,
 				     &rcar_du_vsp_plane_helper_funcs);
 
-		if (type == DRM_PLANE_TYPE_PRIMARY)
-			continue;
-
-		drm_plane_create_alpha_property(&plane->plane);
-		drm_plane_create_zpos_property(&plane->plane, 1, 1,
-					       vsp->num_planes - 1);
+		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;