Engines retain their settings when control from the cockpit is lost. Thrust isn't changed.
Examples enough. In Tenerife the Panam set the engines on fulll power, and after the collision with cockpit gone they stayed on full power untill the (remaining) engines destroyed themselves. (killing at least one person)
TWA engines remained on thrust when the cockpit was cut off.
Both were 747s, but in Lockheed electra's or even A380 the same happens. The Quantas A380 lost control of the engine 1 by severed cables after an uncontained engine failure and they couldn't shut it down. This accident was discussed at EASA, including the difficulty to evacuate an aircraft when engines remain running out of control, but no new safety measures were suggested.
So I am pretty sure the same applies to the 777.
In case of MH17, it is not unlikely that the left engine suffered an uncontained engine failure, given that the forward ring of the intake was found at the cockpit crash site. It is possible that it ingested some radome parts or worse that could have destroyed the fan disc. It would have lost thrust altogether while the right engine would still function. Without countersteering from the cockpit the plane would enter a spin.