backends/drm: don't reset damage with direct scanout

@nicolasfella while I can't reproduce your bug I'm relatively certain that this should fix it.

I think I added this line to make KWin repaint the whole output when direct scanout ends but it does the exact opposite - it makes it not repaint some areas that it should repaint. Buffer age and damage tracking for our render surface are completely independent of actual presentation so we can just remove it