st/mesa: Treat vertex inputs absent in inputMapping as zero in mesa_to_tgsi
After updating vertex inputs being read based on optimized NIR, they may go out of sync with inputs 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 inputs as zero because there is no pass to entirely get rid of them and they don't contribute to shader's output. Fixes: d684fb37 Closes: mesa/mesa#2815 Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com> Reviewed-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com> Reviewed-by: Marek Olšák <marek.olsak@amd.com> Part-of: <mesa/mesa!4705>