drm/amdgpu: fix a vm_flush fence leak

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

Fixes: 41d9eb2c5a2a ("drm/amdgpu: add a fence after the VM flush")
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
index 06f2432..968c426 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
@@ -1758,5 +1758,6 @@
 		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);
 	}
 }