ACPI: Don't send KEY_UNKNOWN for random video notifications

I have a machine here that's sending 0xD1 notifications on the video
device once every second or so. I have no idea why (it's a prototype,
it may be broken), but sending KEY_UNKNOWN is unhelpful and results in
the console becoming unusable. Let's not report keys unless we have
something useful to say about them.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>