drm/exynos: iommu: remove unused entries from exynos_drm_private strcuture

This patch removes unused entries from exynos_drm_private strcuture.
da_start/da_space_size were only used in drm_create_iommu_mapping()
function and never set to other value than the defaults. Instead use
default values directly in arm_iommu_create_mapping() call.

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