intel: Produce a "constrained" output from gen_get_urb_config()

intel: Produce a "constrained" output from gen_get_urb_config()

When calculating a URB configuration, we start with a notion of how
much space each stage /wants/ (to achieve the maximum amount of
concurrency), but sometimes fall back to giving it less than that,
because we don't have enough space.  (Typically, this happens when
the per-stage size is large, or there are many stages, or both.)

We now output a "constrained" boolean which is true if we weren't
able to satisfy all the "wants" due to a lack of space.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8721>