This removes the block at the end of fragment processing and allow
multiple scenes to be queued to the rasterizer up to MAX_SCENES.
Running heaven with this shows up to 39 scenes been allocated in the
first few renders, but it also removes all stalls in rendering until
the present stalls for completion. It reduces frame rendering times
(not enough to make it > 1 fps) but looks to be about 10% increase.
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14923>