drm/exynos/decon5433: kill DECON_UPDATE workaround

Since fixing CMU code (drm/exynos/decon5433: fix CMU programming)
DECON started behave predictable and does not need special care
during DECON_UPDATE writes.

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