formats: Use a hash table for _mesa_format_from_array_format

formats: Use a hash table for _mesa_format_from_array_format

Going through the for loop every time has noticable overhead.  This fixes
things up so we only do that once ever and then just do a hash table lookup
which should be much cheaper.

v2 Jason Ekstrand <jason.ekstrand@intel.com>:
 - Use once_flag and call_once from c11/threads.h instead of pthreads

Reviewed-by: Neil Roberts <neil@linux.intel.com>