drm/nouveau/tmr: fix corruption of the pending list when rescheduling an alarm

drm/nouveau/tmr: fix corruption of the pending list when rescheduling an alarm

commit 9fc64667ee48c9a25e7dca1a6bcb6906fec5bcc5 upstream.

At least therm/fantog "attempts" to work around this issue, which could
lead to corruption of the pending alarm list.

Fix it properly by not updating the timestamp without the lock held, or
trying to add an already pending alarm to the pending alarm list....

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>