Bug#878221: linux-image-4.9.0-4-amd64: Screen flickers randomly with Radeon R9 270X since upgrade from 4.9.0-3 to 4.9.0-4

[besy]

As a side note: I still have some older kernel version from
stretch-backports on my PC. I tried it with the oldest one of these
kernels. The problem doesn't occur with the kernel "4.12.0-0.bpo.2-amd64
#1 SMP Debian 4.12.13-1~bpo9+1 (2017-09-28) x86_64 GNU/Linux".