util: move check for AVX512

util: move check for AVX512

As a side effect solve:
```
[91/1401] Compiling C object src/util/libmesa_util.a.p/u_cpu_detect.c.o
../src/util/u_cpu_detect.c: In function '_util_cpu_detect_once':
../src/util/u_cpu_detect.c:889:11: warning: 'regs2[2]' may be used uninitialized [-Wmaybe-uninitialized]
  889 |       if (((regs2[2] >> 27) & 1) && // OSXSAVE
      |           ^~~~~~~~~~~~~~~~~~~~~~
../src/util/u_cpu_detect.c:823:16: note: 'regs2[2]' was declared here
  823 |       uint32_t regs2[4];
      |                ^~~~~
```

Reviewed-by: Yonggang Luo <luoyonggang@gmail.com>
Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23164>