drm/i915: RGB quantization range stuff

[ville.syrjala at linux.intel.com]

This series tries fix and improve the 'Broadcast RGB' property on i915.

Currently the property takes has two values 'Full' and 'Limited 16:235'.
I'm adding a third option 'Automatic' that automagically changes between
full and limited range based on CEA-861 and DP1.2a rules.

I'm also adding support for informing the display about the currently
used RGB quantization range using the AVI inforframe Q bits. Unfortunately
my TV doesn't support this mechanism, so this part is untested territory.

While doing this I noticed that we're not setting the PR and VIC information
in the AVI infoframe intel_sdvo, while we do set them in intel_hdmi. I wonder
if there's some real reason for this discrepancy?