gallivm: fix up size queries for dx10 sviewinfo opcode

gallivm: fix up size queries for dx10 sviewinfo opcode

Need to calculate the number of mip levels (if it would be worthwile could
store it in dynamic state).
While here, the query code also used chan 2 for the lod value.
This worked with mesa state tracker but it seems safer to use chan 0.
Still passes piglit textureSize (with some handwaving), though the non-GL
parts are (largely) untested.

v2: clarify and expect the sviewinfo opcode to return ints, not floats,
just like the OpenGL textureSize (dx10 supports dst modifiers with resinfo).
Also simplify some code.

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