Screen does not turn back on after turning off for inactivity, but suspend works fine

I have just installed 16.04 on a HP Compaq nc6000. In System Settings -> Details, the graphics line says "Gallium 0.4 on ATI RV350". The relevant line in lspci: "01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] RV350/M10 / RV366660/M11 [Mobility Radeon 9600 (Pro) / 9700]"

The problem:
If the screen tus off due to inactivity, it will not come back on.

I can only force it back on by closing the lid and going into suspend, then waking from suspend, because suspend works fine.

Right now my workaround is to set "Tu screen off when inactive for: Never" but I'd like a better workaround or fix. Please help me troubleshoot this.