gallium/driconf: Allow the driver to parse the driconf options.

This will give the driver a chance to set a device name separate from the
driver name, using info probed during screen creation.  All drivers
querying driconf in screen creation now have to call parsing on their own,
but other drivers get fallback parsing after screen creation.

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12135>