freedreno: last_fence optimization for TC async flushes

freedreno: last_fence optimization for TC async flushes

Without deferred flushes, we would track the last_fence any time when
there had be no rendering/etc since the last flush, to avoid creating
empty batches simply for the purpose of getting a fence.  We can do the
same thing with deferred flushes, where the fence is created from the
frontend thread before we know what will be flushed, by repopulating
the pre-created fence.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10444>