drm/bochs: atomic: use atomic set_config helper

Conversion to atomic modesetting, step four.
Use atomic set_config 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/20190111053752.4004-7-kraxel@redhat.com