displaylink: manually activate dlm.service

The shell script coming with the vendor-provided udev rule simply
starts dlm.service (and sets up some symlinks), and stops dlm.service if
that was the last card plugged in.

On NixOS, some of the cat/grep/sed commands are not available, causing
the script to fail.

Turns out, the symlinks aren't needed at all. Archlinux ships their own
script
(https://aur.archlinux.org/cgit/aur.git/plain/udev.sh?h=displaylink),
which only starts and stops dlm.service, depending on whether there's
cards left or not.

We can further optimize this by simply starting dlm.service on the first
card, and not stopping it at all. Considering dlm won't get stopped if
one of multiple cards is unplugged, it seems to handle disconnects.

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