anv: Emit a NULL vertex for zero base_vertex/instance

anv: Emit a NULL vertex for zero base_vertex/instance

If both are zero (the common case), we can emit a null vertex buffer
rather than emitting a vertex buffer with zeros in it.  The packing of
the VERTEX_BUFFER_STATE is faster because no relocation is emitted and
we can avoid creating the vertex buffer which means one less
anv_state_stream_alloc.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>