drm/radeon/kms: never combine LVDS with another encoder

When linking multiple encoders to a connector, make sure
to not link LVDS with another connector.  Some bioses
have the same i2c line for LVDS and VGA.

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