drm/ttm: add debugfs entry to test pool shrinker v2

Useful for testing.

v2: add fs_reclaim_acquire()/_release()

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/414955/