vt(4): Run vt_timer only when there's something to draw

[dumbbell]

Summary

The goal is to only schedule vt_timer when the screen needs a refresh.

Do you see missing places where the timer should be restarted?

Test Plan

I use the following DTrace script to see when the timer runs:

#!/usr/sbin/dtrace -s

#pragma D option quiet
#pragma D option bufsize=16m
#pragma D option dynvarsize=16m

fbt:kernel:vt_flush:entry
{
  printf(".");
}

[ray]

Agree, too.
Save world using less electric power :)

[ray]

Looks good!

[dumbbell]

Thanks, committed in r270707!