ARM: tegra: Drop display controller parent clocks on Tegra124

The parent clocks are determined by the output that will be used, not by
the display controller that drives the output. On previous generations a
simple RGB output used to be part of the display controller and hence an
explicit parent clock needed to be assigned to the display controller to
drive the RGB output. Starting with Tegra124, that RGB output has been
dropped and the parent clock can therefore be removed from the display
controller device tree nodes.

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