drm/radeon: fix XFX quirk

drm/radeon: fix XFX quirk

commit 1ebf169ad4dc68f18cc0dab35163b0f324fc6c41 upstream.

Only override the ddc bus if the connector doesn't have
a valid one.  The existing code overrode the ddc bus for
all connectors even if it had ddc bus.

Fixes ddc on another XFX card with the same pci ids that
was broken by the quirk overwriting the correct ddc bus.

Reported-by: Mehdi Aqadjani Memar <m.aqadjanimemar@student.ru.nl>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>