drm/amdgpu: just suspend the hw on pci shutdown

We can't just reuse pci_remove as there may be userspace still
doing things.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98638
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97980


Reviewed-by: Christian König <christian.koenig@amd.com>
Reported-and-tested-by: Mike Lothian <mike@fireburn.co.uk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>