drm/nouveau: Make clock gate support conditional

The recently introduced clock gate support breaks on Tegra chips because
no thermal support is enabled for those devices. Conditionalize the code
on the existence of thermal support to fix this.

Fixes: b138eca6

 ("drm/nouveau: Add support for basic clockgating on Kepler1")
Cc: Martin Peres <martin.peres@free.fr>
Cc: Lyude Paul <lyude@redhat.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>