Swapping a ttm object which has no backend pages makes no sense.
Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: xinhui pan <xinhui.pan@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210521083112.33176-1-xinhui.pan@amd.com
CC: stable@kernel.org
Signed-off-by: Christian König <christian.koenig@amd.com>