Hibernation broken since commit 274ad65c9d02 ("drm/radeon: hard reset r600 and newer GPU when hibernating.")

[Sven Joachim]

Recently I got myself a new laptop with the following integrated GPU:

00:01.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Mullins [Radeon R3 Graphics] (rev 40)

I found that hibernation is broken in Linux 4.7+ (it works in Linux 4.6)
and bisected it to commit 274ad65c9d02 ("drm/radeon: hard reset r600 and
newer GPU when hibernating.").

This has already been reported three months ago, but for a much older
GPU, see the thread starting at
https://lists.freedesktop.org/archives/dri-devel/2016-June/110050.html.
The symptoms are exactly the same as described by Boris Petkov in that
thread: after "systemctl hibernate" the screen goes blank, but the
machine remains powered on and needs to be power-cycled.

Any suggestions would be welcome.

Cheers,
       Sven