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>
1 job for modesetting-conn_id in 1 minute and 24 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #61211
job

00:01:24