Today, we stream the compute shader thread IDs simply because they're
(annoyingly) relative to dynamic state base address.  We could upload
them once at compile time, but we'd need a separate non-streaming
uploader for IRIS_MEMZONE_DYNAMIC, and I'm not sure it's worth it.

stream_state pins the buffer for use in the current batch, but also
returns a reference to the pipe_resource.  We dropped this reference
on the floor, leaking a reference basically every time we dispatched
a compute shader after switching to a new one.

The reason it returns a reference is so that we can hold on to it and
re-pin it in iris_restore_compute_saved_bos, which we were also failing
to do.  So if we actually filled up a batch with repeated dispatches to
the same compute shader, and flushed, then continued dispatching, we
would fail to pin it and likely GPU hang.