drm/exynos: mixer: properly update all planes on the same vblank event

This patch also moves mixer_vsync_set_update() to newly introduced
mixer_atomic_begin/flush callbacks. This ensures that all mixer planes
will be updated on the same vsync event.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>