drm/ttm: re-add debugfs tt_shrink file

That got lost when we moved back to a static limit.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210409130113.1459-2-christian.koenig@amd.com