drm/imx: don't call disable_plane in plane destroy path

When the destroy path is called the plane should already be
disabled. If not, this is a core bug and should not be worked
around in the driver.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>