drm/imx: ipuv3-crtc: Use the callback ->atomic_disable instead of ->disable

Now that the drm atomic core supports the callback ->atomic_disable,
we may replace the legacy one ->disable with it.

Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Peter Senna Tschudin <peter.senna@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>