max_payload is limited by the space we have in
drm_dp_mst_topology_mgr::vcpi_mask,payload_mask. We need to track
max_payloads+1 IDs in these masks, see drm_dp_mst_assign_payload_id().
Add a sanity check for this.

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>