drm/exynos/iommu: fix return value check in drm_create_iommu_mapping()

In case of error, function arm_iommu_create_mapping() returns
ERR_PTR() and never returns NULL. The NULL test in the return
value check should be replaced with IS_ERR().
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_iommu.c

@@ -56,8 +56,8 @@ int drm_create_iommu_mapping(struct drm_device *drm_dev)
 	mapping = arm_iommu_create_mapping(&platform_bus_type, priv->da_start,
 						priv->da_space_size,
 						priv->da_space_order);
-	if (!mapping)
-		return -ENOMEM;
+	if (IS_ERR(mapping))
+		return PTR_ERR(mapping);
 
 	dev->dma_parms = devm_kzalloc(dev, sizeof(*dev->dma_parms),
 					GFP_KERNEL);