drm/nouveau: nv84+: fix fence context seqno's

This fixes a regression introduced by "drm/nouveau: rework to new fence interface"
(commit 29ba89b2).

The fence sequence should not be reset after creation, the old value is used instead.
On destruction the final value is written, to prevent another source of accidental
wraparound in case of a channel being destroyed after a hang, and unblocking any other
channel that may wait on the about-to-be-deleted channel to signal.

I'm nothing if not optimistic about any hope of recovery from that. ;-)

Reported-by: Ted Percival <ted@tedp.id.au>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Tested-by: Ted Percival <ted@tedp.id.au>
Acked-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>