turnip: New register packing macros

Emma Anholt requested to merge anholt/mesa:tu-pack into master

This is a port of the new freedreno register packing macros to turnip, and a conversion of tu_cmd_buffer.c to it. I think it results in much more readable code, and gives us a bunch of sanity checks (bitfield sizes, register orders). Passes the same set of CTS tests as before for me.

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