drm: exynos: g2d: do not get to the dma_address directly.

Go though sg_dma_address to get the scatterlist dma address.
drm: exynos: gem: harden exynos_gem_map_sgt_with_dma.

Return EIO if we get an error or zero from dma_map_sg.
Previously zero nents would return success.
drm: exynos: g2d: check for aligned userptr start and size.
drm: exynos: g2d:  check for contiguity of the dma addresses.

To avoid return a dma address to a non contiguous set of buffers
if not under iommu mapping facilities.