headergen2: fix radix
a3xx: add LOD_BIAS

As determined by guesswork and piglit tex-miplevel-selection test..
fortunately there were not many possible values.  Radix of 6 made
positive lod-bias work.  4.6 failed for negative lod-bias but 5.6 and
6.6 worked.  I guess bit 11 is ignored.