drm/tegra: hdmi: Add OPP support

The HDMI on Tegra belongs to the core power domain and we're going to
enable GENPD support for the core domain. Now HDMI driver must use
OPP API for driving the controller's clock rate because OPP API takes
care of reconfiguring the domain's performance state based on HDMI clock
rate. Add OPP support to the HDMI driver.

Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>