Input: wacom - isolate input registration

Although this better co-locates input registration logic,
the main goal is to make it easier to optionally create
input devices or delay creation to later time periods.

Signed-off-by: Chris Bagwell <chris@cnpbagwell.com>
Tested-by: Jason Gerecke <killertofu@gmail.com>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>