drm/radeon/kms: fix tv-out on avivo asics

digital underscan support regressed tv-out.

fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=29985



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