Xi: return AlreadyGrabbed for key grabs > 255

We can't have high keycodes because everything in XKB relies on 8 bits. XI2's
API allows for 32-bit keycodes so we have to take those but nothing in the
server is really ready for this. The effect of this right now is that any high
keycode grab is clipped to 255 and thus ends up grabbing a different key
instead.

https://bugzilla.redhat.com/show_bug.cgi?id=1697804

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit f4cdbf64)
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