drm/amdkfd: Use ordered workqueue to restore processes

Restoring multiple processes concurrently can lead to live-locks
where each process prevents the other from validating all its BOs.

v2: fix duplicate check of same variable

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>