st/mesa: use pipe_sampler_view_release() in st_destroy_context_priv()

Fixes another case of sampler views being created by one context,
shared by another, then deleted by the first, leaving a dangling
pipe context pointer.

Reviewed-by: José Fonseca <jfonseca@vmware.com>

src/mesa/state_tracker/st_context.c

@@ -247,7 +247,7 @@ static void st_destroy_context_priv( struct st_context *st )
    st_destroy_drawtex(st);
 
    for (i = 0; i < Elements(st->state.sampler_views); i++) {
-      pipe_sampler_view_reference(&st->state.sampler_views[i], NULL);
+      pipe_sampler_view_release(st->pipe, &st->state.sampler_views[i]);
    }
 
    if (st->default_texture) {