drm/i915/bdw: Create a separate BDW rps enable

drm/i915/bdw: Create a separate BDW rps enable

This is mostly what we have for HSW with the exceptions of:
no writes:
  GEN6_RC1_WAKE_RATE_LIMIT
  GEN6_RC6pp_WAKE_RATE_LIMIT
  GEN6_RC1e_THRESHOLD
  GEN6_RC6p_THRESHOLD
  GEN6_RC6pp_THRESHOLD

GEN6_RP_DOWN_TIMEOUT - use 1s instead of 1.28s

Don't try to overclock, or program ring/IA frequency tables since we
don't quite have sufficient docs yet.

NOTE: These values do not reflect the changes made recently by Chris.
Since we have no evidence yet what the proper way to tweak for this
platform is, I think it is good to go, and can be optimized by Chris, or
whomever, later.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Drop spurious hunk and drop TODO - having per-platform rps
register frobbing code is in my opinion preferred, now that all the
infrastructure functions are extracted.]
Reviewed-by: Jesse Barnes <jbarnes@virtuosugeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>