drm/nouveau/tegra: Skip manual unpowergating when not necessary

On Tegra186, powergating is handled by the BPMP power domain provider
and the "legacy" powergating API is not available. Therefore skip
these calls if we are attached to a power domain.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>