systemd: more udev clean ups

systemd: more udev clean ups

The udev trigger calls that are there, are actually
spurious.  udev will coldplug those subsystems at
start up anyway, so doing it explicitly is wrong.

This commit drops those trigger calls.

What does matter is the udevadm settle call. It's
what makes things block until the graphics driver
is loaded.

udevadm settle is a big sledgehammer, though. It blocks
until all the triggered events (even stuff unrelated to
graphics) are finished.

This commit adds a --exit-if-exists argument to udevadm settle,
so it will bail early as soon as the graphics devices
are up.