softpipe/draw: fix vertex id in soft paths.

softpipe/draw: fix vertex id in soft paths.

This fixes the vertex id fetch in the non-llvm drawing paths.

This vertex id in elt mode comes from the elts not just a linear
value.

Note we don't bad basevertex in the elts case as it's already included
in the elts by the looks of it (at least tests fail if I add it)

Fixes piglit end-primitive tests and some others.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>