Hello,

> AC: core 0 MHz memory 0 MHz

means your GPU is actually suspended: since you have an Optimus configuration,
the NVIDIA GPU will suspend automatically if it hasn’t received any work to do
in a short while. Try for example to run `DRI_PRIME=1 glxgears`, keep it
running and reissue the cat command: you should not see actual clocks for on
the AC line.
Have a look at https://nouveau.freedesktop.org/wiki/Optimus/ to learn how to
configure Prime.

As for the system hang when trying to reclock a suspended GPU, patches have
been sent to the Nouveau mailing list, but haven’t been merged yet. You can
find them here:
https://github.com/karolherbst/nouveau/commits/clk_cleanup_for_real

> And when i launch a game (Tyranny), it does not work.

What is not working? If you didn’t specify anything, the game should run on the
“primary” GPU by default, which quite often tends to be the Intel GPU.