drm/tegra: dc: Do not register DC without primary plane

Tegra194 contains a fourth display controller that does not own any
windows. Therefore, we cannot currently assign a primary plane to it
which causes KMS to eventually crash. Do not register the display
controller if it owns no windows to work around this.

Note that we still have to enable and probe the display controller
because for some reason all display controllers need to be powered
(and/or clocked) before any registers can be accessed in any of the
display controllers.

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