drm/nouveau/tegra: Don't leave GPU in reset

On Tegra186 systems with certain firmware revisions, leaving the GPU in
reset can cause a hang. To prevent this, don't leave the GPU in reset.

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