drm/exynos: add runtime pm support for mixer

This patch adds support for runtime power management for
drm mixer driver.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Shirish S <s.shirish@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>