HDMI stereo support v4

[Damien Lespiau]

Next installment of the HDMI stereo 3D series, following:
  http://lists.freedesktop.org/archives/dri-devel/2013-September/044885.html

Quite a few changes here, and it's starting to "feel right":
  - We now expose each stereo layout as a separate mode, distinct from the 2D
    mode, instead of piggybacking on the 2D mode flags.
  - 2 bugs fixed:
    - We also expose 3D layouts of modes with alternate clocks
    - The VIC field in the AVI infoframe is now correctly set to the
      corresponding 2D mode
  - We forbid the use of frambuffers with 2 buffers to not set any precedent
    before we can explore the interactions of such framebuffers with the rest
    of the DRM API
  - Rename SET_CAP to SET_CLIENT_CAP