Reiplace HDMI unplug polling with HPD line reading.

Reiplace HDMI unplug polling with HPD line reading.

It looked as if this didn't work at the HW level, but it actually does.
Polling cost us about 100mW if an HDMI monitor was attached.
This also responds more quickly to unplugging.

Based on upstream change:
http://lists.freedesktop.org/archives/intel-gfx/2012-December/023314.html

BUG=chromium-os:37674
TEST=HDMI hotplug works and responds immediately
Change-Id: I9e88671b1ac0e6204feda2666608f30427c8bb79
Reviewed-on: https://gerrit.chromium.org/gerrit/41474
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Tested-by: Stuart Abercrombie <sabercrombie@chromium.org>
Commit-Queue: Stuart Abercrombie <sabercrombie@chromium.org>