llvmpipe: fix multisample point rendering.

Fixes one case in
dEQP-VK.rasterization.primitives_multisample_4_bit.no_stipple.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 d932720ff7dd8fafbb200d9f91c3956d5f0e775e)
2 files changed