drm/amdgpu: add amdgpu_sync_clone

Cloning a sync object is useful for waiting for a sync object
without locking the original structure indefinitely, blocking
other threads.

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