mesa/vbo: Fix handling of attribute 0 in non-compatibilty contexts

mesa/vbo: Fix handling of attribute 0 in non-compatibilty contexts

It is only in OpenGL compatibility-style contexts where generic
attribute 0 and GL_VERTEX_ARRAY have a bizzare, aliasing relationship.
Moreover, it is only in OpenGL compatibility-style contexts and OpenGL
ES 1.x where one of these attributes provokes the vertex.  In all other
APIs each implicit call to glArrayElement provokes a vertex regardless
of which attributes are enabled.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Robert Bragg <robert@sixbynine.org>
Cc: "9.0 9.1 9.2" <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=55503
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66292
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67548