drm/tegra: dpaux: Make VDD supply optional

The VDD supply is only needed to supply power to eDP panels connected to
DPAUX. Technically that supply should be dealt with in the panel driver,
but for backwards-compatibility we need to keep this around anyway.

Also as a bit of background: the reason for why this supply is attached
to DPAUX is to make sure the panel is properly powered early on so that
it can generate a hotplug pulse at the appropriate time. This may no
longer be required given the support for deferred fbdev setup that was
"recently" introduced in DRM/KMS.

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