They use the same register interfaces, so we can simply enable the
existing code on IVB.

v2:
  - resolve conflict with ring freq scaling, we can enable it too
v3:
  - resolve conflict again, this time on drm-intel-next
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NKeith Packard <keithp@keithp.com>