You can take the bus...but it will be time consuming. If you finish eating at 7:30, it could take until close to 8:30 to get to DTD. And that's getting to the first stop. If you stay on the bus, and get off at the last stop (also the second stop), it will take an addtl 10 mins or so, between waiting for guests to board at the Marketplace stop, the drive over to the next stop. Then it's going to take about 8 mins to walk to Cirque.
If it were me, I would just take a cab. The cab can drop you right close to the theater area. And no need to worry about being late. I know that when going to Cirque, they tell you to be there at least 30 mins before...there is some good stuff happening before the show truly starts.