drm/radeon: reset i2c valid to avoid incorrect tv-out polling.

We really don't want to be polling tv-out but since we weren't forcing the
i2c lines to invalid (tv-out has no DDC), we were adding tv connectors to the
polling setup and this was causing blinking on secondary displays.

This fixes the regression Torsten reported.

Reported-by: Torsten Kaiser <just.for.lkml@googlemail.com>
Tested-by: Torsten Kaiser <just.for.lkml@googlemail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>