drm/exynos: IS_ERR() vs NULL bug

of_iomap() doesn't return error pointers, it returns NULL on error.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>