clk: tegra20/30: Don't pre-initialize displays parent clock

Both Tegra20 and Tegra30 are initializing display's parent clock
incorrectly because PLLP is running at 216/408MHz while display rate is
set to 600MHz, but pre-setting the parent isn't needed at all because
display driver selects proper parent anyways.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>