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: 079e5f73 ("mesa/st: rewrite src var when lowering tex_src_plane")