i965/bdw: Fix 3DSTATE_VF_INSTANCING when the edge flag is used

i965/bdw: Fix 3DSTATE_VF_INSTANCING when the edge flag is used

When the edge flag element is enabled then the elements are slightly
reordered so that the edge flag is always the last one. This was
confusing the code to upload the 3DSTATE_VF_INSTANCING state because
that is uploaded with a separate loop which has an instruction for
each element. The indices used in these instructions weren't taking
into account the reordering so the state would be incorrect.

v2: Use nr_elements instead of brw->vb.nr_enabled so that it will cope
    when gl_VertexID is used.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91292
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Tested-by: Mark Janes <mark.a.janes@intel.com>