drm/tegra: Add tegra_dc_setup_clock() helper

This is a small helper that performs the basic steps required by all
output drivers to prepare the display controller for use with a given
encoder.

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

diff --cc drivers/gpu/drm/tegra/dc.c
Simple merge

diff --cc drivers/gpu/drm/tegra/drm.h
Simple merge