drm/modes: Fix CVT-R modeline generation

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=16651



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