drm/radeon: fix XFX quirk

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>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>