drm/tegra: sor: Fix AUX device reference leak

In the case where the AUX provides an I2C-over-AUX DDC channel, a
reference is taken on the AUX parent device of the DDC channel rather
than the DDC channel like it would be for regular I2C controllers. To
make sure the correct reference is dropped, move the unreferencing code
into the SOR driver and make sure not to drop the I2C adapter reference
in that case.

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