Input: wacom - merge out and in prox events

Process out and in prox events for Graphire and Tablet PC devices
in the same loop to simplify the data parsing logic.

[re-applying after revert since other patches are based on this one]

Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>