Skip connectors that do not have an HPD pin.

Should fix:
https://bugs.freedesktop.org/show_bug.cgi?id=39027Signed-off-by: NAlex Deucher <alexdeucher@gmail.com>
Tested-by: NRafał Miłecki <zajec5@gmail.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>