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>
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