gallivm: set non-existing values really to zero in size queries for d3d10

gallivm: set non-existing values really to zero in size queries for d3d10

My previous attempt at doing so double-failed miserably (minification of
zero still gives one, and even if it would not the value was never written
anyway).
While here also rename the confusingly named int_vec bld as we have int vecs
of different sizes, and rename need_nr_mips (as this also changes out-of-bounds
behavior) to is_sviewinfo too.

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