ci: disable freedreno CI on 20.0
We need to repurpose all the chezas to bare-metal testing as we extend master to doing trace testing and more VK testing. I honestly thought the docker db410cs had already all been transitioned already. Unfortunately, backporting all the CI changes from master to 20.0 looks painful. We haven't been doing much backporting in freedreno land, so hopefully this doesn't risk too much regression for these boards.