radeon_hd: Add Northern Island registers
radeon_hd: Fix DisplayPort link training

* Original changes by Bill Randle
* Includes a large number of modifications
  and cleanups.
* Add a "currentMode" to the gDisplay to
  enable easier checking of intended changes.
  (we pass the display_mode around quite a
   bit, adding a "currentMode" allows code
   to know the intended display mode being set
   without passing the mode pointer around as
   much)