drm/exynos: add pm_runtime to FIMD

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 suspended varable to check the suspend status.

Changelog v2:
- Remove unnecessary changes which removed commit callback from decon drivers
  and modify CONFIG_PM_SLEEP -> CONFIG_PM

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