I have heard from some friends that you can take a long distance bus to Jiuzhaigou from Chongqing.

You needn't go to Chengdu. You can take a direct bus to Jiuzhaigou from CQ Main Bus Station which is near Main Railway Station, or from the bus station on the Southern Square of North Railway Station.
The bus departs from the former at 8:00, and from the latter at 8:40. The fare is CNY 216 or 218. The trip takes about 11 hours.