drm/mgag200: Don't do full cleanup if mgag200_device_init fails

Running mgag200_driver_unload when the driver init fails early on
causes functions like drm_mode_config_cleanup to be called. The
problem is, drm_mode_config_cleanup crashes because the corresponding
init hasn't happend yet. There really isn't anything to cleanup after
mgag200_device_init, so we can just pass the error code upwards.

Acked-by: Julia Lemire <jlemire@matrox.com>
Signed-off-by: Christopher Harvey <charvey@matrox.com>
Acked-by: Paul Menzel <paulepanter@users.sourceforge.net>
Signed-off-by: Dave Airlie <airlied@gmail.com>