CHROMIUM: drm: exynos: optional DP hotplug on a GPIO

CHROMIUM: drm: exynos: optional DP hotplug on a GPIO

Allow to setup the Display Port hotplug detection on a vanilla GPIO
instead of the dedicated DP_HPD pin.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
BUG=chrome-os-partner:14491
TEST=boot on Spring, see the display coming up automatically.

Change-Id: I8eaba89bff0fcdc56d05abf769d7097df05807e1
Reviewed-on: https://gerrit.chromium.org/gerrit/36825
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Commit-Ready: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>