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

When using atomic updates the encoder .enable() and .disable() helper
operations are preferred over the (then legacy) .prepare() and .commit()
operations. Implement .enable() and .disable() and rework .prepare(),
.commit() and .dpms() as wrappers around .enable() and .disable(),
easing their future removal.

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