drm/xe: Guard page-fault worker with runtime PM check

In the Linux kernel, the following vulnerability has been resolved:

drm/xe: Guard page-fault worker with runtime PM check

During VM teardown, the VM's runtime PM reference is dropped
asynchronously, allowing the device to autosuspend while stale page
faults belonging to the now-dead VM are still queued. When the
page-fault worker later tries to ack one of these, it calls into
guc_ct_send_locked() on an already-suspended device, tripping:

  Assertion `!xe_pm_runtime_suspended(xe)` failed!
  WARNING at xe_device.c:1267 xe_device_assert_mem_access+0x11c/0x140 [xe]

A live VM/exec queue always holds a PM reference while it has
outstanding work, so if the device is suspended at ack time, the
owning context is already gone and the fault is stale.

Take a runtime PM reference across the entire pagefault
queue worker to safely deliver acks for torn-down VMs.

v3:
- Move PM ref to the generic xe_pagefault_queue_work using
guard(xe_pm_runtime)(xe) instead of tracking it in the GuC
backend(Matt Brost).

v2:
- Hold PM ref across the entire batch (begin/end) instead of per-ack.
This prevents the device from autosuspending mid-batch, which would
leave write_only acks written but the end flush skipped, and skip
counter++, desyncing the cadence check.(Himal)
- Add a comment explaining stale faults.(Himal)

(cherry picked from commit fcc2431d2213dc4d04250c4f1ae87d9c3ae0d455)
[Rodrigo: Added xe_device struct for compatibility while cherry-picking]