drm/exynos: atomic phase 1: use drm_plane_helper_disable()

The atomic helper to disable planes also uses the optional
.atomic_disable() helper. The unique operation it does is calling
.win_disable()

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Tested-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>