[Intel-gfx] Port the i915 HDMI infoframe code to the common infrastructure

[Damien Lespiau]

It's time to revive Thierry and Paulo effort to port the i915 HDMI infoframe
code to the common helpers.

Last attempt was:
  http://lists.freedesktop.org/archives/dri-devel/2013-February/035202.html

I've split the series a bit to address Daniel's concerns about reviewability
and pimped up video/hdmi by adding a new generic union hdmi_infoframe.

Basic testing with both a TV and a monitor seems to indicate we're still
writing the same infoframe data in the DIP registers as before, yey!

-- 
Damien