Should fix the dEQP-VK.synchronization.basic.fence.* flakes.

Change-Id: I9995af6bdb25907b97848f1fc249f6fb8ee15e7d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39051Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>