gallivm: make sampling more robust when the sampler setup is bogus

gallivm: make sampling more robust when the sampler setup is bogus

Pure integer formats cannot be sampled with linear tex / mip filters. In GL
such a setup would make the texture incomplete.
We shouldn't rely on the state tracker though to filter that out, just return
all zeros instead of dying in the lerp.

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