ci: Disable Xorg's screensaver entirely

I think the screensaver kicks in at 10 minutes and obscures the screen, independent of dpms. This causes piglit tests to get flaky (swaps start taking a whole second, and swapbuffersmsc-divisor-zero times out at exactly the wrong time) and slow if the run takes longer than 10 minutes.

Hopefully with this we'll see some piglit glx flakes go away forever, it did seem to for this test locally.

(@ajax does this seem right?)