Fixups to underscan implementation
This is a follow-up from https://bugzilla.gnome.org/show_bug.cgi?id=767804
From the original report:
We ship these patches in Endless. The first one to support different values of the underscan atom is only really required in conjunction with a patched Intel driver that exposes a different value for the "underscan" atom. But it seems a good idea and a nice cleanup to have it upstream too. The second patch streamlines the way the underscan borders are computed and stored, and should make the experience smoother on platforms which have support for the "on" value of the "underscan" atom.