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") Reviewed-by:Marek Olšák <marek.olsak@amd.com> Part-of: <mesa/mesa!9812> (cherry picked from commit 6298347e)