drm/exynos: split exynos_crtc->dpms in enable() and disable()

To follow more closely the new atomic API we split the dpms()
helper into the enable() and disable() helper to get exactly the
same semantics.

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