drm/tegra: dc: Silence RGB output deferred-probe error

Driver fails to probe with -EPROBE_DEFER if display output isn't ready
yet. This produces a bit noisy error message in KMSG during kernel's boot
up on Tegra20 and Tegra30 because RGB output tends to be probed earlier
than a corresponding voltage regulator driver.

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