drm/amdgpu: Do not evict VRAM on APUs with disabled HIBERNATE

Improve commit d796d844 (drm/radeon/kms: make hibernate work on IGPs) to
only migrate VRAM objects if the Linux kernel is actually built with
support for hibernation (suspend to disk).

The better solution is to get the information, if this is suspend or
hibernate, from `amdgpu_device_suspend()`, but that’s more involved, so
apply the simple solution first.

Link: https://bugs.freedesktop.org/show_bug.cgi?id=107277


Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>