Hey guys,
I have the same problem with a 2015 Macbook Pro with AMD R9 370X graphics card.
I found two solutions to the problem.
i) plug on a monitor via HDMI
ii) in Settings / Energy (OS X settings; don't know the exact terms, my system runs in german) there is a option to automatically switch between Intel and AMD graphics card. Deactivate it.
Both work in my case. Hope I could help.