drm/i915: replace use of system_unbound_wq with system_dfl_wq

drm/i915: replace use of system_unbound_wq with system_dfl_wq

Currently if a user enqueue 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.

system_unbound_wq should be the default workqueue so as not to enforce
locality constraints for random work whenever it's not required.

This patch continues the effort to refactor worqueue APIs, which has
begun with the change introducing new workqueues:

commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")

The old system_unbound_wq will be kept for a few release cycles.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Reviewed-by: Krzysztof Karas <krzysztof.karas@intel.com>
Link: https://patch.msgid.link/20251104100032.61525-2-marco.crivellari@suse.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>