iris: Add support for TCS passthrough

iris: Add support for TCS passthrough

If no TCS is provided, create a "passthrough" TCS that will take the
default values set in the API as constants and pass to the TES, along
with any other inputs it expects.  The code to create the NIR shader
is the same as in i965.

Tested with

    ./piglit run -t 'tess' quick_shader r

and fixed a dozen crashes from that list.