drm/radeon/kms: try to detect tv vs monitor for underscan

When enabling underscan for hdmi monitors, attempt to detect
whether we are driving a TV or a monitor.  The should hopefully
prevent underscan from being enabled on monitors attached via
hdmi that do not overscan the image.  Only enable underscan
if the mode is a common hdtv mode (480p, 720p, etc.).

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