Based on:
ca39b1c2 ("user32: Don't report mirrored slave monitors in EnumDisplayMonitors.")
2affb854 ("user32: Change slave to a more neutral word.")

Which seem to got lost during:
31867340 ("win32u: Move NtUserEnumDisplayMonitors implementation from user32.")

Fixes regression with Elite Dangerous launcher freezing when cloned
displays are present.
Signed-off-by: Arkadiusz Hiler <ahiler@codeweavers.com>