drm/nouveau/lib: add null backend

For the moment, just used to speed up vbios-only testing.  Have some
ideas for extending in the future.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/nouveau/nvif/client.c b/drivers/gpu/drm/nouveau/nvif/client.c
index 3c4df1f..3f7ac5b 100644
--- a/drivers/gpu/drm/nouveau/nvif/client.c
+++ b/drivers/gpu/drm/nouveau/nvif/client.c
@@ -62,6 +62,7 @@
 #else
 	&nvif_driver_drm,
 	&nvif_driver_lib,
+	&nvif_driver_null,
 #endif
 	NULL
 };
diff --git a/drivers/gpu/drm/nouveau/nvif/driver.h b/drivers/gpu/drm/nouveau/nvif/driver.h
index ac4bdb3..8bd39e6 100644
--- a/drivers/gpu/drm/nouveau/nvif/driver.h
+++ b/drivers/gpu/drm/nouveau/nvif/driver.h
@@ -17,5 +17,6 @@
 extern const struct nvif_driver nvif_driver_nvkm;
 extern const struct nvif_driver nvif_driver_drm;
 extern const struct nvif_driver nvif_driver_lib;
+extern const struct nvif_driver nvif_driver_null;
 
 #endif