drm/amdkfd: Clean up process queue management

Removed unused num_concurrent_processes.

Implemented counting of queues in QPD. This makes counting the queue
list repeatedly in several places unnecessary.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>