drm/edid: Strengthen the algorithm for standard mode codes

If you have 1920x1200 in both detailed (probably RB) and standard
variants, you probably only want the RB version.  But we have no way of
guessing that from standard mode parse.  So, if a mode already exists
for a given w/h/r, skip adding it.

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