drm/exynos: iommu: improve a check for non-iommu dma_ops

DRM Exynos driver is relying on dma-mapping internal structures when used
with IOMMU enabled. This patch partially hides dma-mapping internal things
by using proper get_dma_ops/set_dma_ops calls.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>