drm/nouveau: fix lock unbalance in nouveau_crtc_page_flip

Fixes a regression introduced by d5c1e84b
"drm/nouveau: hold mutex while syncing to kernel channel".

Cc: stable@vger.kernel.org #3.13
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>