preempt fence restarting

Anyone calling enable_signalling on a preempt fence will need to manually queue a revalidation and restart of the vm. But since the preempt fences are disguised dma-fences and allowed everywhere there is a chance someone might preempt an engine without restarting it.

i915 suspend fences execute a revalidation / restart just after the fence signals, making sure it happens only once for a single preempt fence. Should we do the same?