drm/xe: fix UAF around queue destruction

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

drm/xe: fix UAF around queue destruction

We currently do stuff like queuing the final destruction step on a random system wq, which will outlive the driver instance. With bad timing we can teardown the driver with one or more work workqueue still being alive leading to various UAF splats. Add a fini step to ensure user queues are properly torn down. At this point GuC should already be nuked so queue itself should no longer be referenced from hw pov.

v2 (Matt B) - Looks much safer to use a waitqueue and then just wait for the xa_array to become empty before triggering the drain.

(cherry picked from commit 861108666cc0e999cffeab6aff17b662e68774e3)