drm/nouveau/tegra: Fix error handling

'iommu_domain_alloc()' returns NULL in case of error, not an error pointer.
So test it accordingly.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>