Without the DISPLAY variable set, OpenCL shouldn't be able to find your GPU(s). If no GPU can be found, the only way it could perform the calculations is by using the CPU (to date). Yet, you have `--device gpu` asserted.
It may be worthwhile to find out if MatrixMulImage is falling back to the CPU. How do the figures of the program running in CPU-only mode stack up to GPU-only mode with DISPLAY unset? If they are negligibly close, fallback to CPU would be a reasonable suspicion.
CLInfo still find GPU even without DISPLAY. IMHO it fall back to some default value. setting it to :1 for example show only CPU
second i do not see any difference between :0 :0.0 or none.
Could you please post your system information like following: