staging: vboxvideo: Restore page-flip support

Restore page-flip support now that the atomic conversion is complete.

Since the mode parameter to vbox_crtc_set_base_and_mode() now never
is NULL call drm_atomic_crtc_needs_modeset() to check if we need to
check for input-mapping changes, to avoid doing unnecesarry work on
a flip. And hookup the drm_atomic_helper_page_flip helper to implement
the page_flip callback.

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