anv: fix EXECBUF failure with multiple wait/signal on a semaphore
The delayed submission, due to the submission thread, can lead to a syncobj semaphore used both for wait and signal operations to be reset before the submission thread has sent the syncobj to i915.
This leads to a failure in i915 because we're passing an empty syncobj to wait on.
Signed-off-by: Lionel Landwerlin lionel.g.landwerlin@intel.com Closes: #4945 (closed) Cc: mesa-stable