tnl: Fix green gun regression in xonotic.

Fix an other regression of
mesa: Make gl_vertex_array contain pointers to first order VAO members.
The regression showed up with drivers using the tnl module and
was reproducible using xonotic-glx -benchmark demos/the-big-keybench.dem.

Fixes: 64d2a204
    mesa: Make gl_vertex_array contain pointers to first order VAO members.
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
(cherry picked from commit a6232b69)