drm: rcar-du: Shutdown the display on remove

When the device is unbound from the driver (the DU being a platform
device, this occurs either when removing the DU module, or when
unbinding the device manually through sysfs), the display may be active.
Make sure it gets shut down.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>