DRI_PRIME has no effect when running in a different user

[ghost <i@ghosts.work>]

Hello,

I am trying to run steam using another user, so that there is basic DAC to 
prevent game processes from accessing my other files. I added the user to 
audio/video group and used xhost to allowed it to connect to X. So far so 
good, at least it _runs_.

However, it seems DRI_PRIME has no effect when I am using the game user, 
glxinfo says Mesa is used as OpenGL renderer whereas it is nouveau with my 
regular user.

Someone on #debian@oftc suggested that only users with a "session" get access 
to all hardware, but I have no idea what it might be. Any help is appreciated.

Regards.