drm/tegra: nvdec: Stop channel on suspend

CDMA must be stopped before hardware is suspended. Add channel stopping
to RPM suspend callback. Add system level suspend-resume callbacks.

Runtime PM initialization is moved to host1x client init phase because
RPM callback now uses host1x channel that is available only when host1x
client is registered.

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