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 eeab9c93 but now for outputs. Fixes: d684fb37 Closes: mesa/mesa#3365 Signed-off-by:Danylo Piliaiev <danylo.piliaiev@globallogic.com> Reviewed-by:
Marek Olšák <marek.olsak@amd.com> Part-of: <mesa/mesa!6187>