framebuffer: consolidate calls to udevadm settle

Binary package hint: initramfs-tools

In scripts/init-top/framebuffer we have this code:

 if [ -n "${FB}" ]; then
  # Some framebuffer devices need character devices :-/
  udevadm settle
  MODPROBE_OPTIONS=-q modprobe ${FB} ${OPTS}
  # Wait for the framebuffer devices to be ready
  udevadm settle
 else
  # If we have no graphics devices yet, wait for udev to settle
  [ -d /sys/class/graphics/fbcon ] || udevadm settle
  [ -d /sys/class/graphics/fb0 ] || udevadm settle
  [ -d /sys/class/drm/card0 ] || udevadm settle
 fi

The else clause here appears to be called even on a headless system, which would presumably cause us to call 'udevadm settle' 3 times. Sure, it should be fast (unless there's a bug, which in my case there is), but not as fast as just calling it once. Since I don't see any functional value in calling it each time, I propose we consolidate to one.