drm/xe: Rework preempt fences handling slightly

In order to facilitate a cleaner error handling in the rebind worker, in particular with respect to early userptr invalidation at bind time, split preempt fence handling up in alloc, arm and install, with the idea that arm can be placed under the userptr notifier lock so that we can easily abort and rerun the rebind worker if we hit a userptr invalidation just before arm.

Minimal functional change at this point: Even if someone raced us to rebind, we still check for needed validations and rebinds.

Also add some kerneldoc for the new preempt fence interfaces.

This is a prerequisite for a bigger additional userptr update.

Signed-off-by: Thomas Hellström thomas.hellstrom@linux.intel.com