mesa/st: fix lower_tex_src_plane in multiple samplers scenario

mesa/st: fix lower_tex_src_plane in multiple samplers scenario

"plane[0].i32" is the plane being lowered, it's not the sampler we're looking
for.

It worked when there's a single sampler because, eg for NV12, plane[0].i32 for
the UV plane would be 1 and the added ":uv" sampler would also land at binding
point 1.

Fixes: 079e5f73d72 ("mesa/st: rewrite src var when lowering tex_src_plane")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9812>