the extension is only exposed if a PCIE Aperture that can be used for DirectGMA is enabled. This is not done by default. You need to run the following commands as root and reboot (restarting X is not enough):
This will set the size of the aperture to 96 MB. The max that is currently supported is 128 MB. Now you should see the GL_AMD_bus_addressable_memory extension when calling glxinfo.
Thank you very much. That worked. Do you know whether there is documentation around DirectGMA because I found only in the OpenCL Progragmming Guide v2.7 a tiny bit of information.