Using generic fbdev helpers breaks hibernation

[Alex Deucher]

Maybe someone more familiar with the generic drm fbdev helpers can
help me understand why they don't work with hibernation, at least with
AMD GPUs.  We converted amdgpu to use the generic helpers instead of
rolling our own in this patch[1], but it seems to have broken
hibernation[2].  amdgpu has always set mode_config.prefer_shadow = 1,
but that seems to be the cause of the hibernation breakage with the
generic helpers.  I've been staring at the code for a while now but I
can't see why this fails.  Any pointers?

Thanks,

Alex

[1] - https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=087451f372bf76d971184caa258807b7c35aac8f
[2] - https://bugzilla.kernel.org/show_bug.cgi?id=216119