util/threaded_context: actually invalidate buffers if they aren't busy
checking whether the resource is busy doesn't invalidate the resource, it just notifies whether it's busy. to drop existing data (which is likely just zeroing the valid_buffer_range, but also potentially queuing barrier emission), the invalidate hook needs to be called
Fixes: ec77a2d4 ("gallium/u_threaded: add callbacks and documentation for resource busy checking")