xwayland: Don't create a "fake" crtc for Present

We probably don't want a fake crtc to be visible to clients, and we
definitely don't want to generate events every time we create such a
fake (which would happen as a side effect from RRCrtcCreate hitting
RRTellChanged). As it happens we're not actually using that crtc for
anything because xwayland doesn't store any state on the crtc object,
so it suffices to use the real crtc for the screen.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Tested-by: Roman Gilg <subdiff@gmail.com>
Reviewed-by: Roman Gilg <subdiff@gmail.com>