drm/exynos: use real device for DMA-mapping operations

This patch changes device pointer provided to all calls to DMA-mapping
subsystem from the virtual exynos-drm 'device' to the real device pointer
of one of the CRTC devices (decon, fimd or mixer). This way no more hacks
will be needed to configure proper DMA-mapping address space on the common
virtual exynos-drm device. This change also removes the need for some
hacks in IOMMU related code. It also finally solves the problem of Exynos
DRM driver not working on ARM64 architecture, which provides noop-based
DMA-mapping operations for virtual platform devices.

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