drm/radeon: 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).

Link: https://bugs.freedesktop.org/show_bug.cgi?id=100941


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