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: 079e5f73d7 ("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>

src/mesa/state_tracker/st_nir_lower_tex_src_plane.c

@@ -139,7 +139,7 @@ lower_tex_src_plane_block(nir_builder *b, lower_tex_src_state *state, nir_block
          if (tex_index >= 0 && samp_index >= 0) {
             b->cursor = nir_before_instr(&tex->instr);
 
-            nir_variable* samp = find_sampler(state, plane[0].i32);
+            nir_variable* samp = find_sampler(state, tex->sampler_index);
             assert(samp);
 
             nir_deref_instr *tex_deref_instr = nir_build_deref_var(b, samp);