drm/exynos: fix a wrong error checking

Fix a wrong error checking in exynos_drm_dma.c module.

In the exynos_drm_register_dma function, both arm_iommu_create_mapping()
and iommu_get_domain_for_dev() functions are expected to return NULL as
an error.

However, the error checking is performed using the statement
if(IS_ERR(mapping)), which doesn't provide a suitable error value.
So check if 'mapping' is NULL, and if it is, return -ENODEV.

This issue[1] was reported by Dan.

Changelog v1:
- fix build warning.

[1] https://lore.kernel.org/all/33e52277-1349-472b-a55b-ab5c3462bfcf@moroto.mountain/

Reported-by : Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_dma.c

@@ -107,18 +107,16 @@ int exynos_drm_register_dma(struct drm_device *drm, struct device *dev,
 		return 0;
 
 	if (!priv->mapping) {
-		void *mapping;
+		void *mapping = NULL;
 
 		if (IS_ENABLED(CONFIG_ARM_DMA_USE_IOMMU))
 			mapping = arm_iommu_create_mapping(&platform_bus_type,
 				EXYNOS_DEV_ADDR_START, EXYNOS_DEV_ADDR_SIZE);
 		else if (IS_ENABLED(CONFIG_IOMMU_DMA))
 			mapping = iommu_get_domain_for_dev(priv->dma_dev);
-		else
-			mapping = ERR_PTR(-ENODEV);
 
-		if (IS_ERR(mapping))
-			return PTR_ERR(mapping);
+		if (!mapping)
+			return -ENODEV;
 		priv->mapping = mapping;
 	}