I couldn't find exact info on this on the forums, so thought I would share my experience.
When you arrive in Padang and want to get to Bukit, you can either a) get a DAMRI bus fom the airport to the bus terminal and then get a bigger bus to Bukit or b) negotiate with one on the many shuttle guys outside the airport to get you to Bukit direct. We paid 150,000 for 2 for a shuttle van direct. Toe the airport is about 20km north of the actual Padang city.
As usual, there will be many stop offs along the way in the van, so it may take just as long as the original bus! Good luck!