drm/exynos: implement atomic_{begin/flush} of DECON

Each CRTC's atomic_{begin/flush} must stop/start the update of shadow
registers to active register in the functions. This patch achieves these
purpose by moving the setting of protection bits to those functions from
decon_update_plane.

v2: rebased to the branch exynos-drm-next

Signed-off-by: Hyungwon Hwang <human.hwang@samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>