drm/ttm: fix missing NULL check in ttm_device_swapout

Resources about to be destructed are not tied to BOs any more.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Fixes: 6a9b0289 ("drm/ttm: move the LRU into resource handling v4")
Link: https://patchwork.freedesktop.org/patch/msgid/20220603104604.456991-1-christian.koenig@amd.com