vidcontrol.1: Video buffer dump is syscons only

Fixes: 1f4442f71fa3 (be explicit that -p and -P are sc(4) only)
MFC after: 3 days
Reviewed by: ziaee
Differential Revision: https://reviews.freebsd.org/D53878

(cherry picked from commit 9ec8196f68bac015965164f7f1a65c619bab4e85)