mesa: Use VertexArray, not _VertexArray, in array size expressions.

mesa: Use VertexArray, not _VertexArray, in array size expressions.

Both sizes are VERT_ATTRIB_MAX, so this has no effect.  But it drops a
few trivial uses of the derived state.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Fredrik Höglund <fredrik@kde.org>
Reviewed-by: Brian Paul <brianp@vmware.com>