drm/bochs: atomic: use atomic page_flip helper

Conversion to atomic modesetting, step five.
Use atomic page_flip helper for crtc.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20190111053752.4004-8-kraxel@redhat.com