gallivm: handle unbound textures in texture sampling / texture queries

gallivm: handle unbound textures in texture sampling / texture queries

Turns out we don't need to do much extra work for detecting this case,
since we are guaranteed to get a empty static texture state in this case,
hence just rely on format being 0 and return all zero then.
Previously needed dummy textures (would just have crashed on format being 0
otherwise) which cannot return the correct result for size queries and when
sampling textures with wrap modes using border.
As a bonus should hugely increase performance when sampling unbound textures -
too bad it isn't a useful feature :-).

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