drm/nouveau/imem: support allocations not preserved across suspend

Will initially be used to tag some large grctx allocations which don't
need to be saved, to speedup suspend/resume.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Danilo Krummrich <me@dakr.org>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230919220442.202488-3-lyude@redhat.com