drm/radeon: workaround a hw bug on some radeon chipsets with all-0 EDIDs.

Some RS690 chipsets seem to end up with floating connectors, either
a DVI connector isn't actually populated, or an add-in HDMI card
is available but not installed. In this case we seem to get a NULL byte
response for each byte of the i2c transaction, so we detect this
case and if we see it we don't do anymore DDC transactions on this
connector.

I've tested this on my RS690 without the HDMI card installed and
it seems to work fine.

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