drm/i915/gt: Flush retire.work timer object on unload

drm/i915/gt: Flush retire.work timer object on unload

We need to wait until the timer object is marked as deactivated before
unloading, so follow up our gentle cancel_delayed_work() with the
synchronous variant to ensure it is flushed off a remote cpu before we
mark the memory as freed.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111994
Signed-off-by: Chris Wilson <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Reviewed-by: Tvrtko Ursulin <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]