If we were not able to map the io bar in device init, don't attempt
to unmap it in device fini.  All radeons should have a io bar, so
I doubt this would ever trigger, but just to be on the safe side...

Pointed out by: Alberto Milone <alberto.milone@canonical.com>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>