modesetting: fix conn_id termination and potential overrun by 1 byte
Noticed when porting this logic to xf86-video-nouveau, and valgrind complained about conditional jump based on uninitialized data. Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu> Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>