st/mesa: Treat vertex outputs absent in outputMapping as zero in mesa_to_tgsi

st/mesa: Treat vertex outputs absent in outputMapping as zero in mesa_to_tgsi

After updating vertex outputs being written based on optimized NIR, they may
go out of sync with outputs in mesa IR. Which is translated to TGSI and used
together with NIR if draw doesn't have llvm.

It's much easier to treat such outputs as zero because there is no pass to
entirely get rid of them.

Similar to eeab9c93db84e5759145891e8fdde66a5cdcf917 but now for outputs.

Fixes: d684fb37bfbc47d098158cb03c0672119a4469fe
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3365
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6187>