drm/ttm: nuke ttm_tt_set_(un)populated again

Neither page allocation backend nor the driver should mess with that.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Madhav Chauhan <madhav.chauhan@amd.com>
Link: https://patchwork.freedesktop.org/patch/396948/