drm/imx: fix memory leak when fails to init

Put DRM device on initialization failure path rather than directly
return error code.

Fixes: a67d5088 ("drm/imx: drop explicit drm_mode_config_cleanup")
Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>