drm/nouveau: fix another lock unbalance in nouveau_crtc_page_flip

Fixes a regression introduced by 060810d7ab "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>

drivers/gpu/drm/nouveau/nouveau_display.c

@@ -764,9 +764,9 @@ nouveau_crtc_page_flip(struct drm_crtc *crtc, struct drm_framebuffer *fb,
 	}
 
 	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;