Added the missing part of the FenceSync object
implementation based on the assumption that all
fence operations are always signaled before
calling any sync related wait functions.

Passes all related dEQP tests.

Change-Id: I2fd2ac160dfd1250f55240fc3f696f00579d12d4
Reviewed-on: https://swiftshader-review.googlesource.com/8855Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>