[Intel-gfx] Supporting fused display configurations v2

[Damien Lespiau]

The previous series (and cover letter) can be found at:
http://lists.freedesktop.org/archives/intel-gfx/2013-December/036949.html

Changes:
  - Use SFUSE_STRAP bit 7 in addition to FUSE_STRAP bit 30 to detect fused off
    display,
  - Remove pipe C test for now as we don't know of any device that uses it,
  - Add a note in patch 5 that it needs testing before applying,
  - Add a note on how the first patch was done to help reviewing it.

-- 
Damien