ac: add load_tess_level() to the abi

ac: add load_tess_level() to the abi

Fixes the following piglit tests in radeonsi:

vs-tcs-tes-tessinner-tessouter-inputs-quads.shader_test
vs-tcs-tes-tessinner-tessouter-inputs-tris.shader_test
vs-tes-tessinner-tessouter-inputs-quads.shader_test
vs-tes-tessinner-tessouter-inputs-tris.shader_test

v2: make use of si_shader_io_get_unique_index_patch()
    via the helper in the previous patch rather than
    shader_io_get_unique_index()

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com> (v1)
Reviewed-by: Marek Olšák <marek.olsak@amd.com>