staging: vboxvideo: Atomic phase 2: Wire up state object handlers

Wire up state object handlers for the crtc-s and the planes, call
drm_mode_config_reset() after creating all the crtc-s and encoders and
remove the legacy drm_helper_disable_unused_functions() call.

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