eeepc-laptop: re-add check for eeepc->backlight == NULL

As Corentin points out, we do not create a backlight device if the ACPI
video driver is able to provide equivalent functionality. So we do need
to check before we try to update the backlight device.

We now ignore brightness events completely if we have not created a
backlight device.  This is slightly more cautious than the original
check.

Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: Len Brown <len.brown@intel.com>