commit 806cbc50 upstream.

Fixes a regression introduced by 060810d7 "drm/nouveau: fix locking
issues in page flipping paths".  chan->cli->mutex is unlocked a second time
in the fail_unreserve path, fix this by moving mutex_unlock down.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>