drm/nouveau: fix locking issues in page flipping paths

b580c9e2 introduced additional problems
while trying to solve issues that became apparent while porting to the
new reservation stuff.

The major problem was that the the previously mentioned patch took the
client mutex earlier than previously, but the pinning of new_bo can
can potentially cause a buffer move, which would result in attempting to
acquire the same mutex again.

This commit attempts to fix that "fix".

Thanks to Maarten for the tips on keeping lockdep happy and cooking :)

Reported-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>