HDMI stereo support v4

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