drm/radeon/kms: fix quirk for MSI laptop

The line mux for the connector in the bios tables
is used for enumerating drm connectors.  Since
this laptop has a quirk where the same line much is
listed for both VGA and LVDS, the connectors get
combined.  Setting the line mux on LVDS to an unused
value prevents both encoders from being combined into
the same connector.  This should fix bko bug 13720.

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