staging: vboxvideo: Remove empty encoder_helper_funcs

All the encoder_helper_funcs are optional, and even setting the
drm_encoder_helper_funcs vtable itself is optional and may be left out
when not using any of the helper funcs, so lets drop all of this.

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