systemd: add --subsystem=pci to udevadm trigger call

systemd: add --subsystem=pci to udevadm trigger call

plymouth-start.service does this sort of hacky
"udevadm trigger" call before doing plymouth show-splash,
to ensure plymouth show-splash is called after the
graphcis subsystem is up.  Because this udev trigger call
only passes --subsystem-match calls with no corresponding
--subsystem call, it will trawl through all subsystems in
/sys rather than just the pci subsystem (where all the matches
are guaranteed to be).

This commit adds --subsystem=pci to stop doing that extra work.

All of this will eventually be replaced with plymouthd either
listening for the udev  events itself (or potentially logind,
if logind gains a way to tag a "graphics" capability on a seat).