drm/radeon/kms/DCE3+: switch pads to ddc mode when going i2c

The pins for ddc and aux are shared so you need to switch the
mode when doing ddc.  The ProcessAuxChannel table already sets
the pin mode to DP.  This should fix unreliable ddc issues
on DP ports using non-DP monitors.

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