drm/tegra: Use u64_to_user_ptr helper

Use the u64_to_user_ptr helper macro to cast IOCTL argument u64 values
to user pointers instead of writing out the cast manually. Also do
some other cleanup with user pointers to make them stand out more
and look cleaner.

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