drm/i915: factor out gen6_update_ring_freq

This is needed by the next patch moving the call out from platform
specific RPM callbacks to platform independent code.

No functional change.

v2:
- patch introduce in v2 of the patchset
v3:
- simplify platform check condition (Ville)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>