amdkfd: use schedule() in sync_with_hw

amdkfd uses cpu_relax() in its sync_with_hw() function. Because cpu_relax() is
defined as 'REP; NOP' on x86_64, it will block the CPU from servicing
IOMMU PPR requests.

This may cause a deadlock, because sync_with_hw() won't be completed
until the PPR request has been served.

Therefore, we need to use schedule() instead of cpu_relax() as it is the
minimum requirement to allow other threads to execute.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>

drivers/gpu/drm/amd/amdkfd/kfd_kernel_queue.c

@@ -25,6 +25,7 @@
 #include <linux/mutex.h>
 #include <linux/slab.h>
 #include <linux/printk.h>
+#include <linux/sched.h>
 #include "kfd_kernel_queue.h"
 #include "kfd_priv.h"
 #include "kfd_device_queue_manager.h"
@@ -274,7 +275,7 @@ static int sync_with_hw(struct kernel_queue *kq, unsigned long timeout_ms)
 				*kq->wptr_kernel, *kq->rptr_kernel);
 			return -ETIME;
 		}
-		cpu_relax();
+		schedule();
 	}
 
 	return 0;