https://bugs.freedesktop.org/show_bug.cgi?id=91841
--- Comment #3 from Heiko Lechner Heiko.Lechner@ruhr-uni-bochum.de --- 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.