blk-throttle: Open code __throtl_de/enqueue_tg()

The __throtl_de/enqueue_tg() functions are only be called by
throtl_de/enqueue_tg(), thus we can just open code them to
make code more readable.

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>