radeonsi/nir: fix input processing for packed varyings

radeonsi/nir: fix input processing for packed varyings

The location was only being incremented the first time we processed a
location. This meant we would incorrectly skip some elements of
an array if the first element was packed and proccessed previously
but other elements were not.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>