postmarketos-mkinitfs: disable ps2mouse module when running directfb

On my librem 5 when using directfb in inirtfs, touch input is really inconsistent... the cursor seems to jump around and it's hard to enter keys in osk-sdl. This is completely 'fixed' by disabling the ps2mouse input module in dfb.

I guess this might cause mice to stop working (?), but I can't think of any situation where someone would run osk-sdl in initfs on a device that has a ps2 mouse but no keyboard (which would make more sense to use for entry than the mouse..)