drm/radeon/kms: don't crash if no DDC bus on VGA/DVI connector.

This is strange - like really really strange, twilight zone of strange.
VGA ports have DDC buses, but sometimes for some reasons the BIOS
says we don't and we oops - AMD mentioned bios bugs so we'll have
to add quirks.

reported on irc by nirbheek and
https://bugzilla.redhat.com/show_bug.cgi?id=554323

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