drm/amdkfd: Remove create_workqueue()

alloc_workqueue replaces deprecated create_workqueue().

create_workqueue has been replaced with alloc_workqueue with max_active
as 0 since there is no need for throttling the number of active work items.

WQ_MEM_RECLAIM has not been set to because kfd_process_wq will not be
used in memory reclaim path.

kfd_process_wq is used for delay destruction. A work item embedded in
kfd_process gets queued to kfd_process_wq and when it executes it
destroys and frees the containing kfd_process and thus itself.

This requires a dedicated workqueue because a work item once queued, may
get freed at any point of time and any external entity cannot
flush the work item. So, in order to wait for such a work item,
it needs to be put on a dedicated workqueue.

kfd_module_exit() calls kfd_process_destroy_wq which ensures that all
pending work items are finished before the module is removed.

flush_workqueue is unnecessary since destroy_workqueue() itself calls
drain_workqueue() which flushes repeatedly till the workqueue
becomes empty.

Hence flush_workqueue has been removed.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>