llvmpipe: move fences from per-bin to per-thread

llvmpipe: move fences from per-bin to per-thread

Rather than inserting an lp_rast_fence command at the end of each
bin, have each rasterizer thread call this function directly once
it has run out of work to do on a particular scene.

This results in fewer calls to the mutex & related functions, but more
importantly makes it easier to recognize empty bins.