drm/nouveau/acpi: return supported DSM functions

Return the set of supported functions to the caller. No functional
changes.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>