driconf: Restore the ability to override driconf with the environment.

driconf: Restore the ability to override driconf with the environment.

I distinctly remember testing this during development, yet clearly deleted
the code.

I pulled the parseValue code back up to the top where it used to be since
we have to use it, rather than just adding a prototype.

Fixes: 8a05d6ffc65d ("driconf: Make the driver's declarations be structs instead of XML.")
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7012>