dri: Allow config options to be passed to the loader through extensions.

dri: Allow config options to be passed to the loader through extensions.

Turns out already we have this nice mechanism for providing optional
things from the driver to the loader, and I was going to have to rename
the public global symbol to avoid conflicts when doing megadrivers.

While the former __driConfigOptions is technically loader interface, this
is the only loader that made use of that symbol.  Continue paying
attention to it if we can't find the new option, to retain compatibility
with old drivers.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>