Input: add input_device_enabled()

A helper function for drivers to decide if the device is used or not.
A lockdep check is introduced as inspecting ->users should be done under
input device's mutex.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Link: https://lore.kernel.org/r/20200608112211.12125-2-andrzej.p@collabora.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>