kms: drop window priority below xcb/xdg-shell
The KMS window module shouldn't be loaded when a display server is running and available from the environment. A user with X11 or Wayland available can still enforce the kms window module by cleaning the environment variable from wayland and X11.
Fix #26471 (closed)
@Courmisch is there a specific priority rule that I should follow? vout display are documented (though not really used it seems) but not window plugins.