drm: rcar-du: Rework CRTC enable/disable for atomic updates

When using atomic updates the CRTC .enable() and .disable() helper
operations are preferred over the (then legacy) .prepare() and .commit()
operations. Implement .enable() and rework .disable() to not depend on
DPMS, easing DPMS removal later on.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>