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>

drivers/gpu/drm/nouveau/nvif/client.c

@@ -62,6 +62,7 @@ nvif_drivers[] = {
 #else
 	&nvif_driver_drm,
 	&nvif_driver_lib,
+	&nvif_driver_null,
 #endif
 	NULL
 };

drivers/gpu/drm/nouveau/nvif/driver.h

@@ -17,5 +17,6 @@ struct nvif_driver {
 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