drm/ttm: Avoid calling drm_ht_remove from atomic context

On recent kernels, calling drm_ht_remove triggers a might_sleep() warning
from within vfree(). So avoid calling it from atomic context. The use-cases
we fix here are both from destructors so there should be no concurrent
use of the hash tables.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>