drm/tegra: dpaux: Add helpers for setting up pads

In preparation for adding pinctrl support for the DPAUX pads, add a
couple of helpers functions to configure the pads and control their
power.

Please note that although a simple if-statement could be used instead
of a case statement for configuring the pads as there are only two
possible modes, a case statement is used because when integrating with
the pinctrl framework, we need to be able to handle invalid modes that
could be passed.

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