platform/x86: intel-vbtn: Support separate press/release events

Currently all key events use autorelease, but this forbids use as a
modifier key.

As all event codes come in even/odd pairs, we can lookup the key type
(KE_KEY/KE_IGNORE) for the key up event corresponding to the currently
handled key down event. If the key up is ignored, we keep setting the
autorelease flag for the key down.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>