Comment # 3 on bug 91841 from Heiko Lechner

I booted the same system on my notebook (I installed Ubuntu 15.04 on an usb
drive) equipped with an "Advanced Micro Devices, Inc. [AMD/ATI] Madison
[Mobility Radeon HD 5650/5750 / 6530M/6550M]" and there it works like a charm.

I attached dmesg and lspci output.

Hope that helps.