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>
(cherry picked from commit 48b1af27)