turnip: Reduce the pipeline's CS allocation a bit.

We don't return unused space to the suballocator, so it's a little useful
to limit how much we overallocate to reduce memory footprint.  I took a
look through the tu_cs_emit_array() calls and accounted for a couple of
them in the variant-specific space calculation, then dropped the base
allocation by factors of 2 until we started throwing asserts.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15038>