blk-throttle: pass around throtl_service_queue instead of throtl_data

throtl_service_queue will be used as the basic block to implement
hierarchy support.  Pass around throtl_service_queue *sq instead of
throtl_data *td in the following functions which will be used across
multiple levels of hierarchy.

* [__]throtl_enqueue/dequeue_tg()

* throtl_add_bio_tg()

* tg_update_disptime()

* throtl_select_dispatch()

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Vivek Goyal <vgoyal@redhat.com>