I don't know if this little documented feature was discussed here before.
In case it hasn't it is worth mentioning:
Due legacy considerations the AMD OpenCL software stack was bound to graphics and users had to enable the X-server in order to enjoy compute.
This is no longer completely true!!
In recent drivers it is possible to use AMD OpenCL when the X-server is down as long as you have root privileges.
(just type 'sudo clinfo' when the X-server is down to see the list of available devices) . This feature is called "console mode".
If the X-server is up then business is as usuall, users have to define an X-display for the GPU (sudo aticonfig --initial --adapter=all) in order to use it for compute.