llvmpipe: refactoring of visibility counter handling

llvmpipe: refactoring of visibility counter handling

There can be other per-thread data than just vis_counter, so pass a struct
around instead (some of our non-public code uses this already and this
difference is a major cause of merge pain).

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