drm/exynos: add pm_runtime to Mixer

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

Changelog v2:
- revive MXR_BIT_POWERED flag to keep current dpms mode.

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