drm: rcar-du: Fix crash when enabling a non-visible plane

The DU driver handles non-visible planes (fully clipped by the display's
boundaries) by considering them as disabled. It thus disables the plane
at the hardware level when the plane is moved off-screen. However, if
the plane was previously disabled and is non-visible when it gets
enabled, the attempt to disable it crashes, as the plane wasn't
previously enabled. Fix it.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

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

@@ -279,7 +279,7 @@ static void rcar_du_vsp_plane_atomic_update(struct drm_plane *plane,
 
 	if (plane->state->visible)
 		rcar_du_vsp_plane_setup(rplane);
-	else
+	else if (old_state->crtc)
 		vsp1_du_atomic_update(rplane->vsp->vsp, crtc->vsp_pipe,
 				      rplane->index, NULL);
 }