drm/exynos: remove unnecessary checking to support iommu

Already drm_iommu_attach_device and drm_iommu_detach_device check
whether support iommu internally, so we don't have to call
is_drm_iommu_supported before call them.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>