drm/nouveau/tmr: calibrate for ns timestamps on init

We previously assumed (incorrectly a lot of the time) that PTIMER would
be programmed at a frequency which'd give its 64-bit timestamps in
nanoseconds.

By programming PTIMER ourselves, we avoid this problem.

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