drm/edid: Fix the HDTV hack.

Standard timings don't let you say 1366.  Both 1360 and 1368 have been
seen in the wild.  So invent a CVT timing for it.  CVT will round 1366 up
to 1368; we'll then manually underscan it.

Split this into two parts, since we need to do something sneaky between
them in the future.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>