drm/exynos: simplify DMA mapping

Moving DMA mapping creation to drm_iommu_attach_device allows to avoid
looping through all components and maintaining DMA device flags.

v2: take care of configurations without IOMMU

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