panfrost: add pan_pack_afbc driconf

This is useful for forcing AFBC-P to be used in applications where it's know to work well. This can significantly reduce memory consumption and bandwidth, leading more applications working in the first place, and also better performance.

I was hoping to find a good publicly available use-case for this, but I kinda ran out of time and need to move on. This is useful on some proprietary applications on some Panfrost enabled platforms that I unfortunately can't disclose details about. So it's useful, but I can't currently prove it. If some panfrost gamers want to play around with it, I'm sure someone can find something useful!