drm/exynos: remove drm_iommu_attach_device_if_possible

Already drm_iommu_attach_device checks whether support iommu internally.
It should clear channels always regardless iommu support. We didn't know
because we can detect the problem when iommu is enabled, so we don't
have to use drm_iommu_attach_device_if_possible and then we can remove
drm_iommu_attach_device_if_possible and clear_channels function pointer.

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>