drm/i915: support overclocking on Sandy Bridge

In some configuration, the PCU may allow us to overclock the GPU.
Check for this case and adjust the max frequency as appropriate.  Also
initialize the min/max frequencies to default values as indicated by
hardware.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>