drm/exynos: add pm_runtime to DP

Let pm_runtime handle the enabling/disabling of the device with
proper refcnt instead of rely on specific flags to track the enabled
state.

Chnagelog v3:
- revive dpms_mode to keep current dpms mode.

Changelog v2:
- no change

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