mesa: Skip updating texgen when not doing fixed function.

Between the previous commit and this one, improves GLBenchmark 2.1
offscreen performance by 0.48% +/- 0.24% (n=22, throttling outliers
removed).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
1 file changed