drm/nouveau: increase reservation sequence every retry

This is temporary until the fence framework can be used. With the
lru/reservation atomicity removal it is possible to see your old
sequence number and the buffer being reserved, leading to erroneously
reporting -EDEADLK.

Workaround it by bumping the sequence number every retry.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>