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>

drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c

@@ -102,7 +102,7 @@ nvkm_device_tegra_probe_iommu(struct nvkm_device_tegra *tdev)
 
 	if (iommu_present(&platform_bus_type)) {
 		tdev->iommu.domain = iommu_domain_alloc(&platform_bus_type);
-		if (IS_ERR(tdev->iommu.domain))
+		if (!tdev->iommu.domain)
 			goto error;
 
 		/*