drm/nouveau/dmem: empty chunk do not have a buffer object associated with them.

Empty chunk do not have a bo associated with them so no need to pin/unpin
on suspend/resume.

This fix suspend/resume on 5.1rc1 when NOUVEAU_SVM is enabled.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Reviewed-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>
Tested-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Cc: nouveau@lists.freedesktop.org
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>