drm/i915: Agressive downclocking on Baytrail

Reuse the same reclocking strategy for Baytail as on its bigger brethren,
Sandybridge and Ivybridge. In particular, this makes the device quicker
to reclock (both up and down) though the tendency now is to downclock
more aggressively to compensate for the RPS boosts.

v2: Rebase
v3: Exclude Cherrytrail as Deepak was concerned that the increased
number of register writes would wake the common powerwell too often.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Deepak S <deepak.s@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>