drm/tegra: dc: Check for valid parent clock

drm/tegra: dc: Check for valid parent clock

Check that the desired parent clock is indeed a valid parent for the
display controller clock. This is purely cosmetic at this point since
the parent clocks are specified in DT and all the currently defined
parents are in fact valid parents of the display controller clock.

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