Make EGLDevice backend work out of the box

At the moment, if you want to use the EGLStreams backend, you need to set a magic environment variable and a kernel module param to enable modesetting in the proprietary NVIDIA driver.

While we can't do anything about the kernel module param, I believe that something can be done about the environment variable. In general, if modesetting is enabled, the user should not set any env vars. Things should just work.