Host methods exist to do at least some of what we need, but we are not
currently pushing replay/cancels through a channel like UVM does as it's
not clear whether it's necessary in our case (UVM also updates PTEs with
the GPU).

UVM also pushes a software method for fault cancels on Pascal, seemingly
because the host methods don't appear to be sufficient.  If/when we want
to push the replay/cancel on the GPU, we can re-purpose the cancellation
code here to implement that swmthd.

Keep it simple for now, until we figure out exactly what we need here.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>