gallium/u_threaded: don't call simplify_draw_info redundantly

gallium/u_threaded: don't call simplify_draw_info redundantly

It's always called for the first and second draw in a row, and if they
can't be merged, the second draw becomes the first draw in the next call,
and simplify_draw_info is called again on that. Thus, it's called twice
on any draw that isn't merged. To prevent that, call it when we add
the draws.

Acked-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18199>