Solía poder apagar la pantalla de la computadora portátil ejecutando xset dpms force off. Sin embargo, ahora que actualicé a Ubuntu 17.10, este es el resultado:
X Error of failed request: BadMatch (invalid parameter attributes)
Major opcode of failed request: 147 (DPMS)
Minor opcode of failed request: 6 (DPMSForceLevel)
Serial number of failed request: 12
Current serial number in output stream: 14