drm/i915: Refactor vlv/chv GPU frequency divider setup

The divider used in the GPU frequency calculations is compatible between
vlv and chv. vlv just wants doubled values compared to chv.

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