drm/tegra: Add guard to avoid double disable/enable of RGB outputs

Add guard to check whether RGB output is already enabled in the way it's
done for HDMI output. Fixes possible hang on trying to disable output twice
(first time during driver probe and second on fb registering).

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>