drm/tegra: Allocate per-engine channel in core code

To avoid code duplication, allocate the per-engine shared channel in
the core code instead. This is the usual channel that all jobs are
submitted to when MLOCKing is not in use. Once MLOCKs are implemented
on Host1x side, we can also update this to avoid allocating a shared
channel when MLOCKs are enabled.

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