[lavapipe] Integrate semaphore logic w/ lvp_pipe_sync.

Change-Id: I3799c55ed76e4e58e1b231491096320a17fae3e3
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1033839
Commit-Queue: John Rosasco <rosasco@google.com>
Reviewed-by: Craig Stout <cstout@google.com>