drm/imx: drm_dev_alloc() returns error pointers

We are checking for NULL here, when we should be checking for error
pointers.

Fixes: 54db5dec ("drm/imx: drop deprecated load/unload drm_driver ops")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>