llvmpipe: fix linear rast samples check.
The checks didn't work for the samples == 0 case, just fix it to use the helper.
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The migration is almost done, at least the rest should happen in the background. There are still a few technical difference between the old cluster and the new ones, and they are summarized in this issue. Please pay attention to the TL:DR at the end of the comment.
The checks didn't work for the samples == 0 case, just fix it to use the helper.