wined3d: Bind sampler objects to the correct texture stage.

On current hardware this generally only makes a difference for vertex
textures. This fixes a regression introduced by commit
c6232e1d11.

dlls/wined3d/state.c

@@ -3717,7 +3717,7 @@ static void sampler(struct wined3d_context *context, const struct wined3d_state
 
             if (sampler)
             {
-                GL_EXTCALL(glBindSampler(sampler_idx, sampler->name));
+                GL_EXTCALL(glBindSampler(mapped_stage, sampler->name));
                 checkGLcall("glBindSampler");
             }
         }