drm/exynos/decon5433: fix CMU programming

DECON_CMU register has reserved bits which should not be zeroed, otherwise
IP can behave strangely and cause IOMMU faults.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>