[Intel-gfx] Supporting fused display configurations v3

[Damien Lespiau]

v3, following up a couple of review comments from Ville:
  http://lists.freedesktop.org/archives/intel-gfx/2013-December/037313.html

Changes:
  - Always use INTEL_INFO() after initialization to access dev_priv->info
    (well, except in the reg macros, where it's just too impractical),
  - Cast the device_info pointer returned by INTEL_INFO() to be const to cast
    potential writes by code using the macro,
  - Renamed num_plane to num_sprites as num_planes would imply we're counting
    the primary plane there, but we're not.

-- 
Damien