drm/tegra: Check for NULL pointer instead of IS_ERR()

iommu_domain_alloc() returns NULL on error, it never returns error
pointers.
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NThierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/drm.c

@@ -36,8 +36,8 @@ static int tegra_drm_load(struct drm_device *drm, unsigned long flags)
 
 	if (iommu_present(&platform_bus_type)) {
 		tegra->domain = iommu_domain_alloc(&platform_bus_type);
-		if (IS_ERR(tegra->domain)) {
-			err = PTR_ERR(tegra->domain);
+		if (!tegra->domain) {
+			err = -ENOMEM;
 			goto free;
 		}