drm/tegra: sor: Avoid timeouts on unplug events

When the SOR is disabled in DP mode as part of an unplug event, do not
attempt to power the DP link down. Powering down the link requires the
DPAUX to transmit AUX messages which only works if there's a connected
sink.

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