draw/vs: partly fix basevertex/vertex id

draw/vs: partly fix basevertex/vertex id

This gets the basevertex from the draw depending on whether
it's an indexed or non-indexed draw.

We still fail a transform feedback test for vertex id, as
the vertex id actually an index id, and isn't getting translated
properly to a vertex id, suggestions on how/where to fix that welcome.

Reviewed-by: Brian Paul <brianp@vmware.com>