Hi Alex,
i made many tests about and understand the issue on 8x slot (black screen with
only pulse cursor) probably is gaven by Mesa package.
I found this issue on 7750 pratically there when radeonsi is loaded it made the
crash of the fence and black screen or totally glitched screen.
I will made the same test i had did for 7xxx serie on 6xxx and will see if i
will have desktop witout mesa and will report here.
in case you can check my tests on 7750 here:
https://bugs.freedesktop.org/show_bug.cgi?id=99859#c14

sorry for my eglish is not really good but im try to explain at my best.