drm/i915: Modesetting code rework

* Video output handling, DPMS support and the modesetting part of the
  driver are now mostly in sync with Linux 3.8.13.

* We now have support for the new Haswell Digital Display Interfaces
  (DDI), which can be configured as DisplayPort, HDMI, DVI or eDP
  (embedded Displayport).
  We also support the Haswell Flexible Display Interface (FDI) used to
  drive VGA ports.

* This blog entry from Daniel Vetter contains more information:
  http://blog.ffwll.ch/2012/08/new-modeset-code.html

* This commit also adds a few unrelated changes which were hard to
  separate from the modesetting code. They are mostly about generic
  Haswell and Baytrail/Valleyview support and bugfixes or workarounds
  for older GPU issues.