If chucking PV panels into landfills were actually a widespread practice, I am sure you could produce some statistics about how much of that has actually occurred.
The fact is that PV panel economic lifetime is way, way longer than reactionary renewable opponents want you to believe. It is not 15 years, or 25 years, it is more like 100-400 years. Nobody needs to decommission them en masse, yet. And even if they suddenly did, it still does not present a disposal issue. Suppose there are 100 million PV panels in California. This is the right order of magnitude for our peak generating capacity. If you took every PV assembly in the entire state, stacked them 50 deep so they were about as high as a man, and just put them in a field, they would not even cover 500 acres. That's less than a square mile. Total non-issue.