drm/udl: properly set active_16 flag in udl_crtc_page_flip(). (v2)

When page flipping, we need to mark the new fb as active and unmark the active
flag for the old fb (if different).

Signed-off-by: Haixia Shi <hshi@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Dave Airlie <airlied@gmail.com>