nir: fix per_vertex_output intrinsic

This is supposed to have both BASE and COMPONENT but num_indices was
inadvertantly set to 1.

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit cc3a88e8)