Input: fix used slots detection breakage

Commit f8ec8949 allowed external callers
use slot dropping logic, unfortunately it also broke external users of
input_mt_is_used() as we stopped incrementing frame count unless input
device was set up to automatically drop unused slots.

Fixes: f8ec8949 ("Input: MT - make slot cleanup callable outside
mt_sync_frame()")
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=83081



Reported-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Tested-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Reviewed-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>