drm/nouveau: fix another lock unbalance in nouveau_crtc_page_flip

Fixes a regression introduced by 060810d7abaabca "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>
diff --git a/drivers/gpu/drm/nouveau/nouveau_display.c b/drivers/gpu/drm/nouveau/nouveau_display.c
index b1547b0..72e1571 100644
--- a/drivers/gpu/drm/nouveau/nouveau_display.c
+++ b/drivers/gpu/drm/nouveau/nouveau_display.c
@@ -764,9 +764,9 @@
 	}
 
 	ret = nouveau_page_flip_emit(chan, old_bo, new_bo, s, &fence);
-	mutex_unlock(&chan->cli->mutex);
 	if (ret)
 		goto fail_unreserve;
+	mutex_unlock(&chan->cli->mutex);
 
 	/* Update the crtc struct and cleanup */
 	crtc->fb = fb;