turnip: fix register_index calculations of xfb outputs

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 8731a1beb72da8f4165e5596519a56d16299c3f0

Will fix provoking vertex tf tests when VK_EXT_provoking_vertex
would be enabled:
 dEQP-VK.rasterization.provoking_vertex.transform_feedback.*

Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11111>