llvmpipe: add mutex lock to lp_fence_signalled

Fixes a flaky failure where a fence was getting
destroyed after checking a fence was signalled, while
another thread was still in the middle of lp_fence_signal.

Bug:457786774
Multiply:cmd-buf-benchmark-test
Change-Id: I37c014036c5757c1dfb23f60a0af59996c06e825
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1414314
Reviewed-by: Josh Gargus <jjosh@google.com>
Commit-Queue: Craig Stout <cstout@google.com>