drm/exynos: update overlay via plane from crtc

There is no any reason to update overlay at crtc directly because the
crtc uses plane. Move its code to plane and call proper functions of
plane from crtc.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>