Gallery2: Fix editor

Fix out-of-order operations while setting up the editor

The buffer's producer is created by the cache pipeline generation,
so verifying that it isn't null before calling compute() is wrong.

This breaks a few devices, and should actually break them all, so
fix it; only check if getProducer() is valid after running setProducer()

Change-Id: Ief8843961b622066c4fa1cf41ca5c5322e177250