xkb: only swap once in XkbSetMap

The server swaps part of the request in _XkbSetMapChecks instead of
SProcXkbSetMap (presumably because walking the XkbSetMap request is hard,
and we don't want to maintain another copy of that code).

Swap the first time _XkbSetMapChecks is called, not the second time.

Signed-off-by: Peter Harris <pharris@opentext.com>