drm: clean dirty memory after device release

In current code we register/unregister connector object by
drm_sysfs_connector_add/remove function.

However under some cases, we need to dynamically register or unregister device
multiple times, so we have to go through register -> unregister ->register
routine.

Because after device_unregister function our memory is dirty, we need to do
clean operation in order to re-register the device, otherwise the system
will crash.  The patch intends to clean device after device release.

Signed-off-by: Ma Ling <ling.ma@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>