drm/tegra: dc: Extend debug stats with total number of events

It's useful to know the total number of underflow events and currently
the debug stats are getting reset each time CRTC is being disabled. Let's
account the overall number of events that doesn't get a reset.

Reviewed-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>