evdev: Used floating point instead of fixed point numbers

Following the API change in libinput, change the uses of fixed point
numbers to floating point numbers.

https://bugzilla.gnome.org/show_bug.cgi?id=731178

clutter/evdev/clutter-input-device-evdev.h

@@ -64,8 +64,6 @@ struct _ClutterInputDeviceEvdev
 
   struct libinput_device *libinput_device;
   ClutterSeatEvdev *seat;
-  li_fixed_t dx_frac;
-  li_fixed_t dy_frac;
 };
 
 GType                     _clutter_input_device_evdev_get_type        (void) G_GNUC_CONST;