drm/mst: Don't ignore the MST PBN self-test result

Otherwise this call would have no effect.

Caught by Coverity.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: David Weinehall <david.weinehall@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>