nir/passthrough_gs: Fix array size

nir/passthrough_gs: Fix array size

Triangle strips with adjacency have 6 vertices input, so we need an array big
enough for all 6 vertices to avoid overflow. Fixes passthrough GS generated for
KHR-GLES31.core.draw_indirect.basic-mode-*-triangle*adj*.

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>