util/ra: spiff out select_reg_callback

util/ra: spiff out select_reg_callback

Add a parameter so the callback can know which node it is selecting a
register for.  And remove the graph parameter, as it is unused by
existing users, and somewhat unnecessary (ie. the callback data could
be used instead).

And add a comment so $future_me remembers how this works.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4071>