drm/nouveau: Add support for SLCG for Kepler2

That's right, there's still more power saving to go! Starting with
kepler 2, nvidia hardware has an additional level of clockgating known
as second level clockgating.  The details of this are not exact, but it
seems to work by waiting for a collection of dependent hardware blocks
to be gated before taking affect. As with the previous series, this
results in another noticeable drop in power consumption and is
programmed in the same manner.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>