drm/nouveau/mmu: don't attempt to dereference vmm without valid instance pointer

Fixes oopses in certain failure paths.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c

@@ -1423,7 +1423,7 @@ nvkm_vmm_get(struct nvkm_vmm *vmm, u8 page, u64 size, struct nvkm_vma **pvma)
 void
 nvkm_vmm_part(struct nvkm_vmm *vmm, struct nvkm_memory *inst)
 {
-	if (vmm->func->part && inst) {
+	if (inst && vmm->func->part) {
 		mutex_lock(&vmm->mutex);
 		vmm->func->part(vmm, inst);
 		mutex_unlock(&vmm->mutex);