drm/nouveau: fix another lock unbalance in nouveau_crtc_page_flip

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.

Cc: stable@vger.kernel.org # v3.11+
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>