staging: vboxvideo: Let DRM core handle connector registering

Registering the connector explicitly right after creation is not necessary
for modesetting drivers, because drm_dev_register already takes care of
this on the core side, by calling drm_modeset_register_all.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>