mi: Don't log the full extension list on disable

When enabling or disabling an extension which is not known to the Xserver, it will log an error message and list the extensions it knows about.

That clutters the logs when the Xserver is Xwayland spawned by the Wayland compositor who doesn't actually know the list of extensions enabled at build time in the Xserver.

Considering that disabling a non-existing extension is a no-op anyway, list all the extensions available only when attempting to enable an extension which the Xserver doesn't know about.