For the purpose of running dEQP tests, for now, fences can be
implemented as noop. Once more complex rendering tests exercise
this, we'll be able to implement and test fences.

Bug b/117835459

Change-Id: I2b54be816c83f7a626108f61bd14eb9c969a2b0f
Reviewed-on: https://swiftshader-review.googlesource.com/c/23068Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>