xserver-nodm-init: use /etc/X11/Xserver from service file instead of Xorg directly

* /etc/X11/Xserver is provided by xserver-common and sets correct DPI
  before starting Xorg (or other xserver)

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>