llvmpipe: pass incoming sample_mask into fragment shader context.

llvmpipe: pass incoming sample_mask into fragment shader context.

This links up the api changing the sample mask to passing it into
the fragment shader.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4122>