radeonsi: explicitly choose center locations for 1xAA on Polaris

radeonsi: explicitly choose center locations for 1xAA on Polaris

Unlike SC, the small primitive filter does not automatically use center
locations in 1xAA mode, so this is needed to avoid artifacts caused by
the small primitive filter discarding triangles that it shouldn't.

As a side effect of how the effective number of samples is now calculated,
this patch also avoids submitting the sample locations for line/poly smoothing
when they're not really needed.

Cc: 12.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>