Created attachment 132657 [details]
xorg log with a bit of a backtrace
Happens with radeonsi on RX 480 when starting X, every time. I only bisected, so no proper debug backtrace, but if necessary I can make one.
Reported in Mesa core because it doesn't look radeonsi specific.
Thanks for the report, fixed in Git:
Author: Michel Dänzer <email@example.com>
Date: Thu Jul 13 01:21:00 2017 -0600
st/mesa: Handle st_framebuffer_create returning NULL