nv50,nvc0: set vertex id base to index_bias

nv50,nvc0: set vertex id base to index_bias

Fixes the piglits which check that gl_VertexID includes the base vertex
offset:
  arb_draw_indirect-vertexid elements
  gl-3.2-basevertex-vertexid

Note that this leaves out the original G80, for which this will continue
to fail. It could be fixed by passing a driver constbuf value in, but
that's beyond the scope of this change.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.3 10.4" <mesa-stable@lists.freedesktop.org>