drm: rcar-du: Don't restart group when enabling plane on Gen3

On Gen3 hardware enabling a VSP plane doesn't change any register that
requires DRES to take effect. Avoid a group restart in that case.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>

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

@@ -560,6 +560,12 @@ void __rcar_du_plane_setup(struct rcar_du_group *rgrp,
 		if (rcdu->vspd1_sink != vspd1_sink) {
 			rcdu->vspd1_sink = vspd1_sink;
 			rcar_du_set_dpad0_vsp1_routing(rcdu);
+
+			/*
+			 * Changes to the VSP1 sink take effect on DRES and thus
+			 * need a restart of the group.
+			 */
+			rgrp->need_restart = true;
 		}
 	}
 }

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

@@ -84,15 +84,6 @@ void rcar_du_vsp_enable(struct rcar_du_crtc *crtc)
 
 	__rcar_du_plane_setup(crtc->group, &state);
 
-	/*
-	 * Ensure that the plane source configuration takes effect by requesting
-	 * a restart of the group. See rcar_du_plane_atomic_update() for a more
-	 * detailed explanation.
-	 *
-	 * TODO: Check whether this is still needed on Gen3.
-	 */
-	crtc->group->need_restart = true;
-
 	vsp1_du_setup_lif(crtc->vsp->vsp, crtc->vsp_pipe, &cfg);
 }