drm/nouveau/nvif: namespace of nvkm accessors (no binary change)

NVKM is having it's namespace switched to nvkm_, which will conflict
with these functions (which are workarounds for the fact that as of
yet, we still aren't able to split DRM and NVKM completely).

A comparison of objdump disassemblies proves no code changes.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv84_fence.c

@@ -213,7 +213,7 @@ nv84_fence_destroy(struct nouveau_drm *drm)
 int
 nv84_fence_create(struct nouveau_drm *drm)
 {
-	struct nouveau_fifo *pfifo = nvkm_fifo(&drm->device);
+	struct nouveau_fifo *pfifo = nvxx_fifo(&drm->device);
 	struct nv84_fence_priv *priv;
 	int ret;