drm/bochs: atomic: switch planes to atomic, wire up helpers.

Conversion to atomic modesetting, step three.
Wire up atomic helpers.  Switch planes to atomic.

We are late to the party, the transitional helpers are gone,
so this can't be splitted into smaller steps any more.

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/20190111053752.4004-6-kraxel@redhat.com