drm/i915: sanity check power well sw state against hw state
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: Imre Deak <imre.deak@intel.com> Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>