drm/dp/mst: fix in MSTB RAD initialization

This fix is needed to support more then two
branch displays, so RAD address consist at
least of 2 elements

Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Mykola Lysenko <Mykola.Lysenko@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org