drm/nouveau/dmem: Fix a NULL vs IS_ERR() check

The hmm_devmem_add() function doesn't return NULL, it returns error
pointers.

Fixes: 5be73b69 ("drm/nouveau/dmem: device memory helpers for SVM")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>