drm/tegra: Support context isolation

For engines that support context isolation, allocate a context when
opening a channel, and set up stream ID offset and context fields
when submitting a job.

As of this commit, the stream ID offset and fallback stream ID
are not used when context isolation is disabled. However, with
upcoming patches that enable a full featured job opcode sequence,
these will be necessary.

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