drm/exynos/iommu: merge IOMMU and DMA code

As DMA code is the only user of IOMMU code both files can be merged.
It allows to remove stub functions, after slight adjustment of
exynos_drm_register_dma. Since IOMMU functions are used locally they
can be marked static.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>