drm/exynos/iommu: remove DRM_EXYNOS_IOMMU Kconfig symbol

DRM_EXYNOS_IOMMU symbol is not configurable, it is always equal to
EXYNOS_IOMMU.

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