gallivm: do per-element lod for lod bias and explicit derivs too

gallivm: do per-element lod for lod bias and explicit derivs too

Except for explicit derivs with cube maps which are very bogus anyway.
Just like explicit lod this is only used if no_quad_lod is set in
GALLIVM_DEBUG env var.
Minification is terrible on cpus which don't support true vector shifts
(but should work correctly). Cannot do the min/mag filter decision (if
they are different) per pixel though, only selecting different mip levels
works.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>