drm/sched: Replace use of system_wq with system_percpu_wq

drm/sched: Replace use of system_wq with system_percpu_wq

In the general workqueue implementation, if a user enqueues a work item
using schedule_delayed_work() the used wq is "system_wq" (per-cpu wq)
while queue_delayed_work() use WORK_CPU_UNBOUND (used when a cpu is not
specified). The same applies to schedule_work() that is using system_wq
and queue_work(), that makes use again of WORK_CPU_UNBOUND.

This lack of consistency cannot be addressed without refactoring the API.
For more details see the Link tag below.

This continues the effort to refactor worqueue APIs, which has begun
with the change introducing new workqueues and a new alloc_workqueue flag:

commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
commit 930c2ea566af ("workqueue: Add new WQ_PERCPU flag")

Use the successor of system_wq, system_percpu_wq, for the scheduler's
default timeout_wq. system_wq will be removed in a few release cycles.

Link: https://lore.kernel.org/all/20250221112003.1dSuoGyc@linutronix.de/
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patch.msgid.link/20251106150121.256367-1-marco.crivellari@suse.com