drm/amdgpu: fix VM leaf walking

Make sure we don't try to go down further after the leave walk already
ended. This fixes a crash with a new VM test.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by:  Rex Zhu Rex.Zhu@amd.com
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c

@@ -542,7 +542,8 @@ static void amdgpu_vm_pt_next_leaf(struct amdgpu_device *adev,
 				   struct amdgpu_vm_pt_cursor *cursor)
 {
 	amdgpu_vm_pt_next(adev, cursor);
-	while (amdgpu_vm_pt_descendant(adev, cursor));
+	if (cursor->pfn != ~0ll)
+		while (amdgpu_vm_pt_descendant(adev, cursor));
 }
 
 /**