drm/radeon: Remove deprecated create_singlethread_workqueue

alloc_workqueue replaces deprecated create_singlethread_workqueue().

Each hardware CRTC has a single flip work queue.
When a radeon_flip_work_func item is queued, it needs to be executed
ASAP because even a slight delay may cause the flip to be delayed by
one refresh cycle.

Hence, a dedicated workqueue with WQ_HIGHPRI set, has been used here
since a delay can cause the outcome to miss the refresh cycle.

Since there are only a fixed number of work items, explicit concurrency
limit is unnecessary here.

Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>