drm/radeon/kms: add workaround for rn50/rv100 servers

Some servers have two VGA ports but only report
one in the bios connector tables.  On these systems
always set up the TV DAC so that it displays properly
even if the bios is wrong.

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