staging: vboxvideo: Drop unnecessary drm_connector_helper_funcs callbacks

vbox_mode_valid always returns MODE_OK, which is also the default if no
mode_valid callback is defined.

vbox_best_single_encoder chains to drm_encoder_find, the drm-core will
call drm_encoder_find itself if there is no best_encoder call back.

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