drm/nouveau/pageflip: kick flip handling out of engsw and into fence

This is all very much a policy thing, and hence will not belong in SW
after the rework.

engsw now only handles receiving the event to say "can flip now" and makes
a callback to perform the actual work.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c b/drivers/gpu/drm/nouveau/nouveau_fence.c
index 9775458..a1835d7 100644
--- a/drivers/gpu/drm/nouveau/nouveau_fence.c
+++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
@@ -54,6 +54,7 @@
 void
 nouveau_fence_context_new(struct nouveau_fence_chan *fctx)
 {
+	INIT_LIST_HEAD(&fctx->flip);
 	INIT_LIST_HEAD(&fctx->pending);
 	spin_lock_init(&fctx->lock);
 }