drm/tegra: dc: Request/free syncpoint at init/exit

syncpoints are resources provided by host1x and their lifetime is tied
to the host1x device. They are not properly reference counted either, so
removing the host1x device before any of its clients causes a use-after-
free error. Adding proper reference counting would be a major enterprise
so work around it for now by requesting and freeing the syncpoint at
init and exit time, respectively. The host1x device is guaranteed to be
around at this point.

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