drm/exynos: remove exynos_plane_dpms

The exynos_plane_dpms function handles enabled flag of exynos plane and
calls internal hw driver callback function for hw overlay on/off. But
it causes state disharmory problem currently and is will be obstacle to
apply atomic operation later to keep non-standard per-plane dpms state
like enabled flag.

Let's remove enabled flag, it just stop to recall internal callback
function but hw drivers can handle it properly. And call internal
callback function directly then we can remove unnecessary
exynos_plane_dpms function

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