drm/radeon/kms: small memory leak in atom exit code

This is an unlikely memory leak, but we may as well fix it.  It's easy
to fix and every static checker will complain if we don't.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>