drm/nouveau/pwr: fix the timers implementation with concurent processes

The problem with the current implementation is that adding a timer improperly
checked which process would time up first by not taking into account how much
time elapsed since their timer got scheduled. Rework the re-scheduling
decision t fix this.

The catch with this fix is that we are limited to scheduling timers of up to
2^31 ticks to avoid any potential overflow. Since we are unlikely to need to
wait for more than a second, this won't be a problem :)

Another possible fix would be to decrement the timeouts of all processes but
it would duplicate a lot of code and dealing with edge cases wasn't pretty
last time I checked.

Signed-off-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>