mmu: flush tlbs before deleting page tables

Even though we've zeroed the PDE, the GPU may have cached the page table,
so we need to flush when deleting them.

Noticed while working on replacement MMU code, but a backport might be a
good idea, so let's fix it in the current code too.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drm/nouveau/nvkm/subdev/mmu/base.c

@@ -241,6 +241,8 @@ nvkm_vm_unmap_pgt(struct nvkm_vm *vm, int big, u32 fpde, u32 lpde)
 			mmu->func->map_pgt(vpgd->obj, pde, vpgt->mem);
 		}
 
+		mmu->func->flush(vm);
+
 		nvkm_memory_del(&pgt);
 	}
 }