drm/imx: don't destroy mode objects manually on driver unbind

Instead let drm_mode_config_cleanup() do the work when taking down
the master device. This requires all cleanup functions to be
properly hooked up to the mode object .destroy callback.

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