drm/radeon/kms: disable underscan by default

Lots of HDMI TVs overscan the incoming image by default.
The underscan option was added as a way to compensate for
that by underscanning the image so that the edges would
not be cut off on an overscanning TV.  However, the TV
provides no way of knowing whether it is overscanning or
not.  If the user has disabled overscan on their TV or has a
TV that does not overscan, you will get black bars around the
edges of your screen.  Prior to the patch we got complaints
and bug reports from users with overscanning TVs, now with
the patch, we get lots of complaints and bug reports from
users with non-overscanning TVs.  There's no good default,
but on average there have been more users complaining about
it being on by default than off, so change it to off.  This
will probably result in a new deluge of overscanning TV user
complaints.  There's no winning.

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