drm/i915: Add power well SW/HW state verification

Verify that the refcount of all power wells match their HW enabled
state at the end of modeset HW state readout.

Also add documentation on how the reference count for each power well is
supposed to be acquired during initialization and HW state readout.

Suggested by Ander.

Cc: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1487345986-26511-6-git-send-email-imre.deak@intel.com