drm/exynos: fix to detach device of iommu

The arm_iommu_detach_device() is a function to detach device of iommu
attached by arm_iommu_attach_device(). The exynos-drm uses
arm_iommu_attach_device() so it should use arm_iommu_detach_device() to
detach device of iommu, not iommu_detach_device().

The drm_release_iommu_mapping() is a function to release mapping of
iommu created by arm_iommu_create_mapping(). It is called by
exynos_drm_unload() so shouldn't be called by drm_iommu_detach_device().

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>