gallium/gallivm: Remove workaround disabling AVX code for newer CPUs

Jan Zielinski requested to merge jzielins/mesa:swr-fix-avx-aw into master

The change enables using full 256-bit AVX and AVX2 instructions on platforms that report this capability. As indicated by the removed comment, the current code was meant for quite an old architecture. Therefore it seems to be irrelevant for modern CPUs.

It also fixes: #2029 (closed)

Edited by Jan Zielinski

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