amdgpu_vm_prt_fini uses "vm->root.base.bo" so it must still be valid when
we call it.

Fixes: b65709a9 ("drm/amdgpu: reserve the root PD while freeing PASIDs")
Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>