gallivm: fix min/mag switchover point for nearest/none mip filter

gallivm: fix min/mag switchover point for nearest/none mip filter

Previously, the min/mag switchover point when using nearest/none mip
filter was effectively -0.5 which can't be right. Looks like new OpenGL
thinks it's ok if it's always 0.0 (older versions required 0.5 in some
cases), let's hope everybody else thinks that's fine too.
Refactor this slightly and get the per-quad/per-pixel min/mag decision
values further down to sampling, though still only the first component
is used yet.
While here also fix code trying to skip lod bias application etc. when
mipfilter is none, as this is still needed for determining min/mag filter.

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