drm: rcar-du: Implement system suspend/resume support

To support system suspend operations we must ensure the hardware is
stopped, and resumed explicitly from the suspend and resume handlers.

Implement suspend and resume functions using the DRM atomic helper
functions.

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