drm/radeon/kms: fix displayport->dvi connector DDC.

It appears that attempting AUXCH DDC breaks the subsequent attempt
to do DDC over the i2c lines, so use the sink type to determine
if we should be doing AUXCH or i2c DDC.

This fixes my DVI monitor plugged into DP->DVI convertor.

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