iris: Defer stream output target space allocation until set time

iris: Defer stream output target space allocation until set time

In the future, Marek is planning to make u_threaded_context call
create_stream_output_target() from a different thread than the main
driver thread, which means that we can't safely use uploaders there.

To prepare for this eventual future, just defer the allocation of
the offset BO 'til later.  It's a very small amount of overhead.

Reviewed-by: Zoltán Böszörményi <zboszor@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8964>