drm/exynos: fix dpms operation for mode set

When we do mode set, the dpms mode should be ON. Don't control dpms in
crtc commit function.

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