drm/ttm: fix invalid NULL deref

The BO might be NULL in this function, use the bdev directly.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reported-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Colin Ian King <colin.king@canonical.com>
Fixes: a1f091f8 ("drm/ttm: switch to per device LRU lock")
Link: https://patchwork.freedesktop.org/patch/msgid/20210325152740.82633-1-christian.koenig@amd.com