vga: rework reduced-horizontal timer activation.
vga: rework reduced-horizontal timer activation.

This avoids triggering TIM1 when it isn't needed, which in turn prevents
DRQs from being noticed and latched somewhere inaccessible.

cbiffle/m4vgalib,cbiffle/m4vgalib,cbiffle/m4vgalib,cbiffle/m4vgalib