drm/nouveau: Introduce NvPmEnableGating option

This adds the NvPmEnableGating config option to nouveau, which can be
used to enable or disable clockgating for supported chipsets. Enabling
can be done by passing

	config=NvPmEnableGating=1

To nouveau. If your chipset supports it, you'll see a message in your
kernel log indicating that clockgating is enabled. Since clockgating has
only had limited testing thus far, we leave this option disabled by
default for now.

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