drm/i915: Move overclocking detection to alongside RPS frequency detection

Move the overclocking max frequency detection alongside the regular
frequency detection, before we expose the undefined value to userspace.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468397438-21226-4-git-send-email-chris@chris-wilson.co.uk