zink: skip updating descriptor buffer sets that aren't active

this is a no-op and illegal

Fixes: 7ab5c5d36d ("zink: use EXT_descriptor_buffer with ZINK_DESCRIPTORS=db")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21085>

src/gallium/drivers/zink/zink_descriptors.c

@@ -961,6 +961,8 @@ zink_descriptors_update_masked_buffer(struct zink_context *ctx, bool is_compute,
       return;
 
    u_foreach_bit(type, changed_sets | bind_sets) {
+      if (!pg->dd.pool_key[type])
+         continue;
       assert(type + 1 < pg->num_dsl);
       assert(type < ZINK_DESCRIPTOR_BASE_TYPES);
       bool changed = (changed_sets & BITFIELD_BIT(type)) > 0;