drm: rcar-du: crtc: Control CMM operations

Implement CMM handling in the crtc begin and enable atomic callbacks,
and enable CMM unit through the Display Extensional Functions
register at group setup time.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
[Fix printk format modifier for size_t variable]
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>