Are you connecting remotely to your linux machine?
Is X-server running?
Try logging into X-server console directly and run "clinfo" from an xterm and see if GPU is visible.
After installing 12.8 sdk version and directly using the laptop instead of sshx connection i can now get the GPU device for openCL !!
There are forum thread that have discussed this issue extensively.
Check this thread ioctl permissions on Linux
(not sure why fonts bloated up.. :-( )
Th basic thing that you need to do is a "xhost +" - to provde remote access control to x-server.
Once you do this, you can get GPU device listed even when you connect remotely.
Usually, we add "xhost +" to the X startup script.
For GDM based desktops, one needs to edit the GDM specific script.
For LightDM, you need to look at LigthDM specifi script.
Here is an excert from the thread above:
$ cat /etc/lightdm/lightdm.conf
$ cat /etc/lightdm/xhost.sh
chmod uog+rw /dev/ati/card*
Make sure that xhost.sh is executable:
sudo chmod a+x /etc/lightdm/xhost.sh