drm/exynos: Implement DPMS_STANDBY for HDMI

drm/exynos: Implement DPMS_STANDBY for HDMI

Enable the "blue screen" when we get a DPMS_STANDBY request for HDMI.
This allows us to turn off the mixer, but maintain audio over HDMI when
we turn the screen idle.

BUG=chrome-os-partner:15709
TEST=Tested on snow by ensuring audio continues to play during idle

Change-Id: I2639c0d1cd7384881b5fd651039fef71b27c76ed
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/38122
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>