drm/ttm: Explain why ttm_bo_add_move_fence uses a shared slot

Motivated because I got confused and Christian confirmed why this
works. I think this is non-obvious enough that it merits a slightly
longer comment.

Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: Christian König <ckoenig.leichtzumerken@gmail.com>
Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210519082409.672016-1-daniel.vetter@ffwll.ch