Buses for everyday travel to and from the parks starts an hour before the parks open. The bus for MK for breakfasts will start no later than 630am. (Character breakfasts buses to other parks generally start around 7am.) I've read from some that it can start earlier. They start earlier for MK because of the earlier breakfast ADR's at monorail resorts. Check with your transportation CM at POP the night before. I believe Trns. CM are dressed in purple. Just let them know that you have 720am ressie at Chef Mickey's and everything should be fine. Worst case scenario, if you miss the early bus, you could call a cab and make it in time that way.
If you catch the 630am bus you should have no problems making it to your ADR.
Be sure to tell your bus driver that you are eating at the Contemporary Resort. If you're lucky and have a really nice driver, they might drop you off a CR instead of making you walk. Don't expect it, but it's a nice bonus if it happens. Ask, and the worst they can tell you is "no."
Enjoy your trip!