drm/i915: Preserve current RPS frequency across init

Select idle frequency during initialisation, then reset the last known
frequency when re-enabling. This allows us to preserve the user selected
frequency across resets.

v2: Stop CHV from overriding the user's choice in cherryview_enable_rps()

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468397438-21226-2-git-send-email-chris@chris-wilson.co.uk