HDMI stereo support v5

[Damien Lespiau]

v4 was:
http://lists.freedesktop.org/archives/dri-devel/2013-September/045340.html

Changes from v4:

  - The kernel is now in charge of adjusting the stereo mode timings.
  - There is a per-connector opt-in boolean to expose stereo modes, letting
    people enable stereo for each driver/connector.
  - The series now depends on the latest bits from the drm-intel tree
  - Removed the 2 buffers check that was judged unnecessary
  - I realized that for frame packing modes (where you have to adjust the
    vdisplay to be 2 * vdisplay + vblank) with a NTSC clock I was
    reconstructing the clock without adjusting it by 1000 / 1001. Now that the
    timing computation is in the kernel, fixed it there.
  - I also checked that we correctly match against the 2D CEA mode for the AVI
    infoframes. The last bit to make it work was the introduction of crtc_clock
    to separate the original clock from the one computed by
    drm_mode_set_crtcinfo()
  - And finally, I booted with i915.fastboot=1 and discovered that the pipe_src
    register was getting clobbered, so fixed that as well