I had some time and since i could reproduce this problem in my virtual machines too i've started to track down the problem.
After a few hours of searching i finally found it. The reason was because dbus wasn't build with the useflag X enabled.
Adding the X useflag to dbus solved the problem.
echo "sys-apps/dbus X" >> /etc/portage/package.use
After creating a bug report on bugs.gentoo.org devs also mentioned that if you're using a desktop/kde profile you shouldn't hit the problem because there dbus would be build with X enabled anyway.
I never though about that but it's true, i'm using a default amd64 profile. The desktop/kde profile do enable to much for me.
Anyway, i also found another issue. ligthdm-kde also need dbus with X enabled otherwise it wont start.