iris: setup EdgeFlag Vertex Element when needed.

iris: setup EdgeFlag Vertex Element when needed.

If Vertex Shader uses EdgeFlag the hardware request that it is setup
as the last VERTEX_ELEMENT_STATE. If SGVS are add at draw time we
need to also reconfigure the last 3DSTATE_VF_INSTANCING so its
VertexElementIndex points to the new Vertex Element that contains
the EdgeFlag.

So if draw parameters or edgeflag are not used the CSO generated at
iris_create_vertex_element is sent directly in the batches. But if
edge flag is used we adjust last VERTEX_ELEMENT_STATE and
last 3DSTATE_VF_INSTANCING using their alternative edge flag version
we generate at iris_create_vertex_element and store at the CSO.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>