drm/radeon/kms: fallback to default connector table

if necessary for combios

Some early combios radeon cards don't have a connector
table or dac table in the bios, if they do not, fallback
to the default tables.

Should fix kernel bug 14963.

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