As karolherbst pointed on the #nouveau irc channel, I patched my distro kernel
sources with this patch : 

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/arch/x86/mm/kmmio.c?id=cfa52c0cfa4d727aa3e457bf29aeff296c528a08

Build with the same options as current kernel.

It did the trick.

I finally was able to do the mmiotrace test.

The test consisted in running the bare X server with only the nvidia-settings
application.

Forced Maximum Performance level.

At reach then forced adaptive.

At reach then exited both application and X.