drm/tegra: Don't leak kernel pointer to userspace

Each open file descriptor can have any number of contexts associated
with it. To differentiate between these contexts a unique ID is required
and back when these userspace interfaces were introduced, in commit
d43f81cb

 ("drm/tegra: Add gr2d device"), the pointer to the context
structure was deemed adequate. However, this leaks information about
kernel internal memory to userspace, which can potentially be exploited.

Switch the context parameter to be allocated from an IDR, which has the
added benefit of providing an easy way to look up a context from its ID.

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