mesa/vbo: Add draw_id field to struct _mesa_prim

mesa/vbo: Add draw_id field to struct _mesa_prim

The drivers will need this for passing in gl_DrawIDARB. For indirect
multidraw calls, we get the prim array and prim[i].draw_id == i and is
redundant. But for non-indirect calls, we get one primitive at a time
and need the draw_id field.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>