Reinstate: llvmpipe: allow vertex processing and fragment processing in parallel

Reinstate: llvmpipe: allow vertex processing and fragment processing in parallel

This reinstates patch: ec8104c6b227421b3a21e9c0652e3050066bb169
 llvmpipe: allow vertex processing and fragment processing in parallel

For now unknown reasons bisecting an error had lead to this patch, but
the actual bug was in virglrendrerer and the accoring feature was now only
enabled with https://gitlab.freedesktop.org/mesa/mesa/-/issues/6130

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6130
Original patch was

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15684>