iris: Dig through the cache to find a BO in the right memzone

iris: Dig through the cache to find a BO in the right memzone

This saves some util_vma thrash when the first entry in the cache
happens to be in a different memory zone, but one just a tiny bit
ahead is already there and instantly reusable.  Hopefully the cost
of a little extra searching won't break the bank - if it does, we
can consider having separate list heads or keeping a separate VMA
cache.

Improves OglDrvRes performance by 22%, restoring a regression from
deleting the bucket allocators in 694d1a08d3e5883d97d5352895f8431f.

Thanks to Clayton Craft for alerting me to the regression.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>