drm/i915: enable ring freq scaling, RC6 and graphics turbo on Ivy Bridge v3

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: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>