CHROMIUM: drm/exynos: Drop non-essential frames on dpms off

CHROMIUM: drm/exynos: Drop non-essential frames on dpms off

This patch drops all frames except for the last pending one when a crtc
is disabled. It will also wait on the last frame to render before
returning from DPMS off.

BUG=chromium-os:37797
TEST=Tested manually:
	- Plug/unplug HDMI with no content on the HDMI monitor
	- Plug/unplug HDMI with animation (sad puppy) on HDMI monitor
	- Plug/unplug HDMI with animation + cursor movement on monitor
	- Suspend/resume with animation on monitor
	- Idle suspend/resume

Change-Id: I3e26e8bac9be0d71cf3c0756d1cc3dd92d2612de
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/41772
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>