drm/ttm: lock resv object during destruction

Calling ttm_bo_cleanup_memtype_use() destroys the TT object
which in turn could result in warnings without this.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/359288