drm/radeon/kms: make sure rio_mem is valid before unmapping it

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>