drm: rcar-du: Shutdown the display on system shutdown

When the system shuts down or warm reboots, the display may be active,
with the hardware accessing system memory. Upon reboot, the DDR will not
be accessible, which may cause issues.

Implement the platform_driver .shutdown() operation and shut down the
display to fix this.

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