drm/tegra: hub: Do not enable orphaned window group

Though the unconditional enable/disable code is not a final solution,
we don't want to run into a NULL pointer situation when window group
doesn't link to its DC parent if the DC is disabled in Device Tree.

So this patch simply adds a check to make sure that window group has
a valid parent before running into tegra_windowgroup_enable/disable.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>