drm/imx: drop deprecated load/unload drm_driver ops

Drop the load/unload driver ops, as they are deprecated because of their
inherent races, with devices being visible to userspace before they are
fully initialized.

Move this code into the driver bind/unbind routines bracketed by the
proper drm_dev_alloc/register and drm_dev_unregister/unref calls.

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