drm/nouveau/core/memory: add reference counting

We need to be able to prevent memory from being freed while it's still
mapped in a GPU's address-space.

Will be used by upcoming MMU changes.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/nouveau/nvkm/engine/xtensa.c b/drivers/gpu/drm/nouveau/nvkm/engine/xtensa.c
index 06bdb67..7054938 100644
--- a/drivers/gpu/drm/nouveau/nvkm/engine/xtensa.c
+++ b/drivers/gpu/drm/nouveau/nvkm/engine/xtensa.c
@@ -86,7 +86,7 @@
 	nvkm_wr32(device, base + 0xd94, 0); /* FIFO_CTRL */
 
 	if (!suspend)
-		nvkm_memory_del(&xtensa->gpu_fw);
+		nvkm_memory_unref(&xtensa->gpu_fw);
 	return 0;
 }