Bug#920759: workaround

[franckr <franckr01@online.de>]

Control: retitle -1 (with workaround) amdgpu+linux-images 4.9 & 4.19 crash randomly & always after X started, when dpm is enabled

Workaround: disable dynamic power management
  ex: put in /etc/default/grub: GRUB_CMDLINE_LINUX_DEFAULT="quiet amdgpu.dpm=0"

Mechanism guess:
As behavior (works or crashes) depends of linux-image package (see below), and seems to evolve randomly across package versions (an older package version works, then a newer crashes, then a newer newer works again):
This suggests that a variable/structure is not properly initialized inside amdgpu dynamic power management code. Then depending of the RAM content during package compilation, a good value (=> package will work) or a bad value (=> will crash) is built-in inside the package.
This could explain the apparent random behavior across package versions.

  Work  = (C) + (B) + linux-image-4.19.0-0.bpo.4-amd64-unsigned 4.19.28-2~bpo9+1 = upstream 10-Mar-2019
  Crash =       (B) + linux-image-4.19.0-0.bpo.4-amd64-unsigned 4.19.28-2~bpo9+1 = upstream 10-Mar-2019
  Work  =       (B) + linux-image-4.19.0-0.bpo.2-amd64-unsigned 4.19.16-1~bpo9+1 = upstream 16-Jan-2019
  Crash =       (B) + linux-image-4.9.0-9-amd64                 4.9.161-1 = upstream 27-Feb-2019
  Crash =       (A) + linux-image-4.9.0-8-amd64                 4.9.144-1 = upstream 08-Dec-2018 
  Work  =       (A) + linux-image-4.9.0-8-amd64                 4.9.135-1 = upstream 20-Oct-2018

  (C) = disabled dynamic power management
  (B) = (firmware-amd-graphics   20190114-1) + (1920x1200 59.95Hz 8bit per RGB = nothing special)
  (A) = (firmware-amd-graphics   20161130-4)

Kind Regards,
Franck