staging: vboxvideo: Move pin / unpin of fb out of vbox_crtc_set_base_and_mode

Move pin / unpin of fb out of vbox_crtc_set_base_and_mode() so that it can
be used to implement the atomic_update drm_plane_helper_func for the primary
plane.

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