mesa: optimize _mesa_attr_zero_aliases_vertex()

mesa: optimize _mesa_attr_zero_aliases_vertex()

After the context is initialized, the API and context flags won't
change.  So, we can compute whether vertex attribute 0 aliases
vertex position just once.

This should make the glVertexAttrib*() functions a little quicker.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>