Input: MT - Handle frame synchronization in core

Most MT drivers perform the same actions on frame synchronization.
Some actions, like dropping unseen contacts, are also unnecessarily
complex. Collect common frame synchronization tasks in a new function,
input_mt_sync_frame(). Depending on the flags set, it drops unseen
contacts and performs pointer emulation.

With init flags and frame synchronization in place, most MT drivers
can be simplified. First out are the bcm5974 and hid-multitouch
drivers, following this patch.

Reviewed-and-tested-by: Benjamin Tissoires <benjamin.tissoires@enac.fr>
Tested-by: Ping Cheng <pingc@wacom.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>