drm/tegra: Check syncpoint ID in the 'submit' IOCTL

In case of invalid syncpoint ID, the host1x_syncpt_get() returns NULL and
none of its users perform a check of the returned pointer later. Let's bail
out until it's too late.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>