gallivm: use the new mip per quad handling in texture fetch path

gallivm: use the new mip per quad handling in texture fetch path

No longer have to split fetching into quads dynamically if mip levels
are not the same for all quads (aos sampling still always splits due
to performance reasons).
Instead handle multiple mip levels further down, minification etc. takes
this into account.

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