blk-mq: Move the "state" debugfs attribute one level down

Move the "state" attribute from the top level to the "mq" directory
as requested by Omar.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>