nir: Use nir_shader_get_entrypoint in TCS quad workaround code.

nir: Use nir_shader_get_entrypoint in TCS quad workaround code.

We want to insert the code at the end of the program.  Looping over
all the functions (of which there was only one) was the old way of doing
this, but now we have nir_shader_get_entrypoint(), so let's use it.

Suggested by Connor Abbott.

v2: Update for nir_shader_get_entrypoint API change.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>