drm/connector: Check for destroy implementation

Connectors need to be cleaned up with a call to drm_connector_cleanup()
in their drm_connector_funcs.destroy implementation.

Let's check for this and complain if there's no such function.

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