drm/amdgpu: Change system_unbound_wq with system_dfl_wq

drm/amdgpu: Change system_unbound_wq with system_dfl_wq

system_wq (per-CPU) and system_unbound_wq (unbound) are the older
workqueue name, replaced by system_{percpu|dfl}_wq.
The new workqueues have been introduced by:

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

Usage of older workqueues will now trigger a pr_warn_once() because they are
marked as deprecated as per commit:

  64d8eae3f895 ("workqueue: Add warnings and fallback if system_{unbound}_wq is used")

So change the used workqueue with the newer, keeping the same behavior.

Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>