sysfs registration/teardown fixups

A check in drm_sysfs_connector_remove was supposed to allow it to be called
even with unregistered objects, to make cleanup paths a little simpler.
However, device_is_regsitered didn't always seem to return what we thought it
would, so we'd sometimes end up leaving objects lying around rather than
unregistering them.

Fix this situation up by requiring devices to be registered before being
removed.  Any problems resulting from this change should be easier to track
down than the alternative (which is leaving kobjects registered after unload).