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>