drm/exynos: split display's .dpms() into .enable() and .disable()

The DRM Core doesn't have a dpms() operation anymore, everything
now is enable() or disable().

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