gschema overrides: Disable dim on idle

We blank quickly so the dim is more confusing than it helps since the
display looks already turned off in bright sunshine and the power saving
are negligible when we blank soon afterwards..
2 jobs for no-dim in 3 minutes and 19 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Package
passed #281619
librem5
build-debian-package

00:00:52

 
  Test Package
passed #281620
librem5
autopkgtest-debian-package

00:02:27