[Mesa-dev] i965 Data Port fixes and clean-ups

[Kenneth Graunke]

Patch 1 fixes the variable indexing Piglit tests on Ivybridge.  I was
hoping it would also fix bug 41318 (rendering issues in GLBenchmark/PRO)
as well, but alas.  Still, seems worthwhile; the old code was clearly wrong.

Patch 2 may be somewhat controversial, and implements a change in behavior
on Sandybridge.  Not observed to help or hurt anything yet.

The rest of the series cleans up and documents the code.

Tested via full Piglit runs on GM45, Ironlake, Sandybridge, Ivybridge (GT2).
Not tested on Broadwater/Crestline because I didn't have one of those handy.