radeon lockups w/ 5.1-R

Hi folks,

anyone else still seeing occasional lockups when using a Radeon
graphics card?  I've had these frequently with a R7500 on 5.0p7
and now installed 5.1 hoping that the issue had been fixed in
the meantime.  It went well for two days but it just fscked up
again so the problem is still there.  It only happens when I
exit the X session.  Display becomes garbled and the machine
locked up completely (didn't always do so in the past with 5.0,
I sometimes could log in over the network and reboot the machine.)
I think the problem was already known for 5.0, or does it warrant
a new PR?

-- 
  Matthias Buelow;  mkb_at_{mukappabeta.de,informatik.uni-wuerzburg.de}