i965: Disable Z16 on contexts that don't require it.

It appears that Z16 on Intel hardware is in fact slower than Z24, so
people are getting surprisingly hurt when trying to use Z16 as a
performance-versus-precision tradeoff, or when they're targeting GLES2 and
that's all you get.

GL 3.0+ have Z16 on the list of required exact format sizes, but GLES
doesn't, so choose the better-performing layout in that case.  Improves
GLB 2.7 trex performance at 1920x1080 by 10.7% +/- 1.1% (n=3) on my IVB
system.

Reviewed-by: Kenneth Graunke <[email protected]>

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