drm: rcar-du: Replace dev_private with container_of

Now that drm_device is embedded in rcar_du_device, we can use
container_of to get the rcar_du_device pointer from the drm_device,
instead of using the drm_device.dev_private field.

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