drm/i915: CHV GPU frequency to opcode functions

Adding chv specific fre/encode conversion.

v2: Remove generic function and platform check (Daniel)

Signed-off-by: Deepak S <deepak.s@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>