iris: Fix export of fences that have already completed.

iris: Fix export of fences that have already completed.

After flushing batches, iris_fence_flush() asks the kernel whether
each batch's last_syncpt has already signalled or not.  (The idea is
that either the compute or render batch may not have actually had any
work queued up, so last_syncpt there might have been signalled a long
time ago.)  If it's already completed, we don't bother to record it.

A strange corner is the case of repeated flushes.  For example, we
might flush for some reason, and hit a glFlush(), and hit SwapBuffers.
It's possible for all the batches to have been flushed previously, -and-
for them to have actually completed.  In this case, we'll see that there
are no syncobj's to wait on, and record fence->count == 0.

This works fine internally - fence_finish can see count == 0 and realize
that it doesn't need to wait, for example.  But when working with native
FDs, we may be asked to export a fence with count == 0.  So we need an
actual synchronization primitive we can hand off.  Because all of the
relevant batches had been signalled when creating the fence, we want the
new dummy fence to be signalled as well.

So we just make a signalled syncobj and export it.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>