Suggested by Daniel.

v2:
- sanitize the state checking condition, the original was rather
  confusing (partly due to the unfortunate naming of
  i915.disable_power_well) (Ville)
- simpler message+backtrace generation by using WARN instead of WARN_ON
  (Ville)
- check if always-on power wells are truly on all the time
Signed-off-by: NImre Deak <imre.deak@intel.com>
Reviewed-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>