HD panel (1366x768) found most commonly on laptops can't be represented
exactly in CVT/DMT expression, which leads to 1368x768 instead, because
1366 can't be divided by 8.

Add a hack to convert to 1366x768 manually as an exception.
Signed-off-by: NTakashi Iwai <tiwai@suse.de>
Acked-by: NAdam Jackson <ajax@redhat.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>