drm/exynos: IOMMU support should not be selectable by user

If system provides IOMMU feature, Exynos DRM should use it by default,
because the Exynos DRM subdrivers don't work correctly when Exynos IOMMU
driver has been enabled and no IOMMU support has been compiled into Exynos
DRM driver.

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