drm/panthor: disable async work during unplug

drm/panthor: disable async work during unplug

A previous change, "drm/panthor: Fix UAF race between device unplug and
FW event processing", fixes a real issue where new work was unexpectedly
queued after cancellation. This was fixed by a disable instead.

Apply the same disable logic to other device level async work on device
unplug as a precaution.

Signed-off-by: Ketil Johnsen <ketil.johnsen@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patch.msgid.link/20251029111412.924104-1-ketil.johnsen@arm.com
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>