drm/exynos: add dpms for hdmi

The power and clocks turns on always in exynos hdmi and mixer driver,
but we should turn off the power and clocks of exynos hdmi and mixer
when the hdmi cable unplugged or when hdmi unused.

There are two interrupt to detect hotplug of hdmi cable - internal
interrupt and external interrupt. The internal interrupt can use only
when hdmi is dpms on so if hdmi is dpms off, we should use external
interrupt to detect hotplug of hdmi cable. If hdmi is dpms on, we cannot
external interrupt because the gpio pin for external interrupt is used
to hdmi HPD pin for internal interrupt.

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>