drm/xe/userptr: Convert invalidation to two-pass MMU notifier

In multi-GPU scenarios, asynchronous GPU job latency is a bottleneck if
each notifier waits for its own GPU before returning. The two-pass
mmu_interval_notifier infrastructure allows deferring the wait to a
second pass, so all GPUs can be signalled in the first pass before
any of them are waited on.

Convert the userptr invalidation to use the two-pass model:

Use invalidate_start as the first pass to mark the VMA for repin and
enable software signalling on the VM reservation fences to start any
gpu work needed for signaling. Fall back to completing the work
synchronously if all fences are already signalled, or if a concurrent
invalidation is already using the embedded finish structure.

Use invalidate_finish as the second pass to wait for the reservation
fences to complete, invalidate the GPU TLB in fault mode, and unmap
the gpusvm pages.

Embed a struct mmu_interval_notifier_finish in struct xe_userptr to
avoid dynamic allocation in the notifier callback. Use a finish_inuse
flag to prevent two concurrent invalidations from using it
simultaneously; fall back to the synchronous path for the second caller.

v3:
- Add locking asserts in notifier components (Matt Brost)
- Clean up newlines (Matt Brost)
- Update the userptr notifier state member locking documentation
  (Matt Brost)

Assisted-by: GitHub Copilot:claude-sonnet-4.6
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260305093909.43623-3-thomas.hellstrom@linux.intel.com

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