drm/exynos: fimd: move window protect code to prepare/cleanup_plane

Only set/clear the update bit in the CRTC's .atomic_begin()/flush()
so all planes are really committed at the same time.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>