drm/radeon/kms: enable underscan option for digital connectors

This connector attribute allows you to enable or disable underscan
on a digital output to compensate for panels that automatically
overscan (e.g., many HDMI TVs).  Valid values for the attribute are:

off - forces underscan off
on - forces underscan on
auto - enables underscan if an HDMI TV is connected, off otherwise

default value is auto.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>