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/include/nvif/client.h

@@ -31,7 +31,7 @@ int  nvif_client_resume(struct nvif_client *);
 
 /*XXX*/
 #include <core/client.h>
-#define nvkm_client(a) ({ \
+#define nvxx_client(a) ({ \
 	struct nvif_client *_client = nvif_client(nvif_object(a)); \
 	nouveau_client(_client->base.priv); \
 })