Add mplexid to dummy entries when scheduling around live TV.

This was missed when the mplexid optimization was done.  Thanks to
pjokinen for spotting it and submitting the patch.

Refs #12032

Signed-off-by: David Engel <dengel@mythtv.org>

(cherry picked from commit 2ebd924706b97015e15f3075f93e03bfc1616fef)