llvmpipe: increase number of queries which can be binned simultaneously to 64

llvmpipe: increase number of queries which can be binned simultaneously to 64

Gallium (but not OpenGL) does allow nesting of queries, but there's no
limit specified (d3d10 has no limit neither). Nevertheless, for practical
purposes we need some limit in llvmpipe, otherwise we'd need more complex
handling of queries as we need to keep track of all binned queries (this
only affects queries which gather data past setup). A limit of 16 is too
small though, while 64 would suffice.

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