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: NLaurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>