i965: get outputs written from nir info

i965: get outputs written from nir info

This is a step towards dropping the GLSL IR version of
do_set_program_inouts() in i965 and moving towards native nir support.

This is important because we want to eventually convert to nir and
use its optimisations passes before we can call this GLSL IR pass.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>