nir+ir3+freedreno: Passthrough TCS support

The first patch patch adds a nir helper to generate a passthrough TCS, and the second switches radeonsi over to use the nir helper. (CI is happy with it, and seems to have piglit coverage which should exercise this path.. @mareko you might want to have a look to make sure I didn't overlook anything since the nir helper differs, but only vary slightly, from the radeonsi code it replaces). Remaining patches wire up ir3 and freedreno support.

Fixes 17 little piglits.