drm/vc4: hdmi: Remove call to drm_connector_unregister()

drm_connector_unregister() is only to be used for connectors that have been
registered through drm_connector_register() after drm_dev_register() has
been called. This is our case here so let's remove the call.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>