(In reply to kristof from comment #4)
> PS : can someone tells me if it this an "abnornal" things in the last dmesg
> | egrep 'drm|radeon' please ?
> 
> Thanks and have a nice day

The kernel side looks fine.  It appears to be a problem with the usermode
stack.  You might be able to work around it by disabling acceleration.  Try
adding:
Option "NoAccel" "True"
to the device section of your xorg config.

I'm not really sure how well the usermode stuff works on BE systems.