draw: make sure viewport index is fetched from leading vertex

draw: make sure viewport index is fetched from leading vertex

Viewport index should only be used on a per primitive basis, so
instead of fetching it from each vertex, potentially making each
vertex in a primitive use a different viewport index, which is
obviously broken, make sure that we only fetch from the first
vertex in the primitive making the viewport index the same
for the entire primtive.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: José Fonseca<jfonseca@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>