drm/exynos: clear channels only when iommu is enabled

This is simplest solution about reported problem[1]. It's no problem to
clear channel only when iommu is enabled, if we consider that we cannot
recognize iommu errors when iommu is disabled and it have been valid
until now. But this cannot be nice solution.

[1] https://lkml.org/lkml/2015/7/21/404



Reported-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>