It has absolutely nothing to do with the GPU. It's a problem with LightDM and SSDs (or some other "abnormally fast" piece of hardware, Nvidia cards cause the same issue)

Try adding

respawn 

below

stop on runlevel

In /etc/init/lightdm.conf

Often times, if you run into this error, you can hit

[Ctrl] + [Alt] + [5] to enter a terminal. Login and try

sudo lightdm start

If that doesn't yield anything try

sudo service lightdm restart

As a side note, my SATAIII SSD was connected to the SATAII port on my logic board when I ran into this issue. Verify that your main SSD is on the correct port before you try anything.