If we are doing an MST transaction and we've gotten HPD and we
lookup the device from the incoming msg, we should take the mgr
lock around it, so that mst_primary and mstb->ports are valid.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>