drm/i915: New offset for reading frequencies on CHV.

Use new Sideband offset to read max/min/gaur freq based on the SKU it
is running on. Based on the Number of EU, we read different bits to
identify the max frequencies at which system can run.

v2: reuse mask definitions & INTEL_INFO() to get device info (Ville)

v3: add break in switch conditions (Ville)

Signed-off-by: Deepak S <deepak.s@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>