staging: vboxvideo: Call drm_atomic_helper_check_plane_state from atomic_check

Extend our planes atomic_check callbacks to be more thorough by calling
the drm_atomic_helper_check_plane_state helper.

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