llvmpipe: fix multisample lines.

This also needs another lines fix, but at least align the code
with tri and points

Cc: "20.3" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7705>
(cherry picked from commit 2c0a078fdb4b856ecc29d5cf9b0478afc2309aa8)
3 files changed