drm/ttm: wrap tt destroy. (v2)

All places this was called was using bo->ttm either direct
or indirectly.

v2: move to ttm_bo

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200915024007.67163-3-airlied@gmail.com