drm/i915: Hide gen6_update_ring_freq()

This function is no longer used outside of intel_pm.c so we can stop
exposing it and rename the __gen6_update_ring_freq() to take its place.

Suggested-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468397438-21226-8-git-send-email-chris@chris-wilson.co.uk