Only take the driver-internal button mapping to count buttons (#23405)

Regression:
    If a user has multiple buttons mapped to the same button number, the
    number of buttons counted is set to a wrong value.  e.g. a button
    mapping of 1 1 1 for a mouse with three buttons would only initialize 1
    button to the X server.

    In the future, the user cannot change this button mapping as the server
    only knows about one button.

The user-supplied button map (option ButtonMapping) shouldn't matter when
counting the buttons. Only the driver-internal mapping (BTN_0 -> button 1,
etc.) matters.

X.Org Bug 23405 <http://bugs.freedesktop.org/show_bug.cgi?id=23405>

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 415b6ffa95)

src/evdev.c

@@ -1544,8 +1544,7 @@ EvdevProbe(InputInfoPtr pInfo)
         int mapping = 0;
         if (TestBit(i, pEvdev->key_bitmask))
         {
-            mapping =
-                pEvdev->btnmap[EvdevUtilButtonEventToButtonNumber(pEvdev, i)];
+            mapping = EvdevUtilButtonEventToButtonNumber(pEvdev, i);
             if (mapping > num_buttons)
                 num_buttons = mapping;
         }