drm/amdgpu: fix a vm_flush fence leak

Looks like .last_flush reference is left at teardown.
Leak reported by CONFIG_SLUB_DEBUG.

Fixes: 41d9eb2c ("drm/amdgpu: add a fence after the VM flush")
Reviewed-by: NChunming Zhou <david1.zhou@amd.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NGrazvydas Ignotas <notasas@gmail.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c

@@ -1758,5 +1758,6 @@ void amdgpu_vm_manager_fini(struct amdgpu_device *adev)
 		fence_put(adev->vm_manager.ids[i].first);
 		amdgpu_sync_free(&adev->vm_manager.ids[i].active);
 		fence_put(id->flushed_updates);
+		fence_put(id->last_flush);
 	}
 }