turnip: Port krh's packing macros from freedreno to tu.

turnip: Port krh's packing macros from freedreno to tu.

This introduces some minor unpacking of the temporary fd_reg_pair structs
to code that previously was packing a whole register field.

In the pack wrapper in tu_cs.h, I added some explanatory docs, dropped the
relocs handling since we don't need it, and removed the extra regs[] in
the __ONE_REG() macro (which was causing gcc's optimizer to fall on its
face in my release build).

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3455>