drm/tegra: dpaux: Configure pads as I2C by default

The DPAUX code paths already configure the pads in AUX mode, but there
is no way to reconfigure them in I2C mode for HDMI (the DPAUX module is
unused in that case). Enabling the pads in I2C mode by default is the
quickest way to support HDMI. Eventually this may need an explicit call
in the user drivers.

Signed-off-by: Thierry Reding <treding@nvidia.com>