drm/tegra: dc: Don't clean up unused display controller

Display controllers that don't own any windows and which therefore
aren't currently being assigned a primary plane are not registered
as a CRTC with the DRM framework. They request neither a syncpoint
nor an interrupt because they don't use them, so avoid cleaning up
those resources.

Signed-off-by: Thierry Reding <treding@nvidia.com>