x11: never enable DPMS if we didn't disable it

Enabling DPMS even though you disabled it globally is pretty unfriendly,
so don't do it. Instead, we only disable DPMS if it was enabled, and
only enable it if we disabled it ourselves.

The other way should never happen (disabling DPMS permanently), unless
mpv crashes during playback.
etc/input.conf: add example how to change window size by key binding

Also drop a vague hint how to do it in the manpage.