[Intel-gfx] Breadcrumbs, again

[Chris Wilson]

We have a major bottleneck in waiting with many clients that is
impacting customer workloads. This is because we wake up every waiter
after the GPU advance for them all to try and identify if they were the
lucky one. The classic thundering herd, and the response is to only wake
the next in the queue who then wakes up all completed clients.

This also provides a low overhead signaling framework that *works*.

(This series follows on the BAT fix, since regressions get priority.)
-Chris