xfree86: Fix interpretation of xf86WaitForInput timeout

Commit aa6717ce switched xf86WaitForInput from using select(2) to using
poll(2). Before this change, the timeout was interpreted as being in
microseconds; afterwards it is fed directly to xorg_poll which interprets
it as being in milliseconds. This results in the function potentially
blocking 1000x longer than intended. This commit scales down the timeout
argument before passing it to xorg_poll, being careful to ensure the result
is not rounded down due to integer division.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 2fbf62b2)