drm/radeon: fix lockup when BOs aren't part of the VM on release

commit 26d4d129 upstream.

If we unmap BOs before releasing them them the intervall tree locks
up because we try to remove an entry not inside the tree.

Based on a patch from Michel Dänzer.
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/gpu/drm/radeon/radeon_vm.c

@@ -1107,7 +1107,8 @@ void radeon_vm_bo_rmv(struct radeon_device *rdev,
 	list_del(&bo_va->bo_list);
 
 	mutex_lock(&vm->mutex);
-	interval_tree_remove(&bo_va->it, &vm->va);
+	if (bo_va->it.start || bo_va->it.last)
+		interval_tree_remove(&bo_va->it, &vm->va);
 	spin_lock(&vm->status_lock);
 	list_del(&bo_va->vm_status);