drm/radeon/kms: reset ddc_bus in object header parsing

Some LVDS connectors don't have a ddc bus, so reset the
ddc bus to invalid before parsing the next connector
to avoid using stale ddc bus data.  Should fix
fdo bug 28164.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>