drm/nouveau/timer: add a way to cancel alarms

Since alarms don't play well with suspend, it is important every alarm
user cancels his tasks before suspending.

The task should be rescheduled on resume.

Signed-off-by: Martin Peres <martin.peres@labri.fr>
Tested-by: Martin Peres <martin.peres@labri.fr>
Tested-by: Dash Four <mr.dash.four@googlemail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>