xserver-common: start Xsession when starting Xserver

* since this commit
  commit 305038f9


  Author: Jonathan Liu <net147@gmail.com>
  Date:   Wed Jun 12 08:30:15 2013 +0000
    xserver-nodm-init: do not execute Xsession twice

  xserver-nodm-init wasn't working correctly when it was used together
  with xserver-common which doesn't start Xsession from Xserver (unlike
  x11-common in oe-core).

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>