Poll for HDMI unplugs in Intel driver.

Poll for HDMI unplugs in Intel driver.

The driver was relying on a successful EDID read following a hotplug interrupt to indicate HDMI sink presence.  This leads to missing HDMI cable unplug events because the DDC lines can remain up, allowing an EDID read to complete, well after the HPD line goes down during unplugging.

Unfortunately the hardware does not seem to allow querying the HPD line directly.

BUG=chrome-os-partner:13495
TEST=HDMI unplugs are now detected.

Change-Id: I82f33809cdd4fca5cf427902641ba678241f6647
Reviewed-on: https://gerrit.chromium.org/gerrit/34121
Tested-by: Stuart Abercrombie <sabercrombie@chromium.org>
Commit-Ready: Stuart Abercrombie <sabercrombie@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>