drm/exynos: fix iommu address allocation order

This patch modifies iommu address allocation order from 64k
to 4k. 64k order causes waste of the io space and this was
our mistakes.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>