drm/exynos: fimd: clear channel before enabling iommu

drm/exynos: fimd: clear channel before enabling iommu

If any fimd channel was already active, initializing iommu will result
in a PAGE FAULT (e.e. u-boot could have turned on the display and
not disabled it before the kernel starts). This patch checks if any
channel is active before initializing iommu and disables it.

Changelog v2:
- consider SoC without SHADOWCON register

Signed-off-by: Akshu Agrawal <akshu.a@samsung.com>
Signed-off-by: Prathyush K <prathyush.k@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>