drm/exynos: Don't touch DPMS when updating overlay planes

DPMS settings should only be changed by a full modeset.
exynos_plane_update() should only care about updating the planes itself
and nothing else.

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