drm/radeon: report disconnected for LVDS/eDP with PX if ddc fails

If ddc fails, presumably the i2c mux (and hopefully the signal
mux) are switched to the other GPU so don't fetch the edid from
the vbios so that the connector reports disconnected.

bug:
https://bugzilla.opensuse.org/show_bug.cgi?id=904417



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