nir/passthrough_gs: Correctly set vertices_in

nir/passthrough_gs: Correctly set vertices_in

If the input primitive has adjacency, the output primitive will have fewer
vertices than the input. For example, if we input TRIANGLE_STRIPS_ADJACENCY, we
need to set vertices_in = 6 even though we'll output TRIANGLE_STRIPS with
vertices_out = 3. Respect that, in order to correctly handle adjacency inputs.

Fixes: ea14579f3dc ("nir: handle primitives with adjacency")
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Antonino Maniscalco <antonino.maniscalco@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24780>