drm: add register and unregister functions for connectors

Introduce generic functions to register and unregister connectors. This
provides a common place to add and remove associated user space
interfaces.

Signed-off-by: Thomas Wood <thomas.wood@intel.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>