drm/exynos/mixer: correct vsync configuration sequence

Specification advises to clear vsync indicator before configuring vsync.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>