staging: vboxvideo: Atomic phase 1: Use drm_plane_helpers for primary plane

Use drm_plane_helpers for the primary plane and replace our custom
mode_set callback with drm_helper_crtc_mode_set.

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