drm/exynos: add PM functions for hdmi and mixer

This patch supports the PM for hdmi and mixer. Turn off hdmi and mixer
when suspended, and when resume, will turn on them by hdmi hotplug
detection if hdmi is attached.

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>