turnip: fix register_index calculations of xfb outputs

nir_assign_io_var_locations() does not use outputs_written when assigning driver locations. Use driver_location to avoid incorrectly guessing what locations it assigned.

Copied from lavapipe 8731a1be

Will fix provoking vertex tf tests when VK_EXT_provoking_vertex would be enabled:

 dEQP-VK.rasterization.provoking_vertex.transform_feedback.*