HID: wacom: Perform all event processing as part of report processing

In some cases, we need access to information before it becomes available
to the 'event' handler. In particular, for some devices we cannot properly
process the finger data without first knowing the "contact count" at the
very end of the report (e.g. the Cintiq 24HDT touch screen, when forced
through the GENERIC codepath).

Since the HID subsystem doesn't provide a way to take action before 'event'
is called, we take a cue from hid-multitouch.c and add a pre-process step
within the 'report' handler that performs the same function.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>