st/mesa: fix nir_load_per_vertex_input parameter

num_components should be 1 as we're loading an offset value.

Fixes: ec68f049 ("st/mesa: switch GL_SELECT shader to IO intrinsics")
Closes: #12774


Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <!33982>