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>