i965: Update comments about Z16 being slow.

We've learned a few things since we originally disabled Z16; this attempts
to summarize the issue.  I am no expert on this subject, though, so the
comment may not be totally accurate.

I did some benchmarking on GM45 and Ironlake, and discovered that for
GLBenchmark 2.7 EgyptHD, using Z16 was 3% slower on GM45 (n=15), and
4.5% slower on Ironlake (n=95).  So, we can drop the "on Ivybridge"
aspect of the comment - it's always slower.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Matt Turner <[email protected]>
Reviewed-by: Chia-I Wu <[email protected]>

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