swr/rast: Fix index buffer overfetch issue for non-indexed draws

swr/rast: Fix index buffer overfetch issue for non-indexed draws

Populate pLastIndex, even for the non-indexed case.  An zero pLastIndex
can cause the index offsets inside the fetcher to have non-sensical values
that can be either very large positive or very large negative numbers.

Reviewed-By: Bruce Cherniak <bruce.cherniak@intel.com>