gpu: host1x: Allow loading tegra-drm without enabled engines

gpu: host1x: Allow loading tegra-drm without enabled engines

Add support to register host1x devices without requiring subdevices.
This ensures syncpoint functionality remains available even when engine
subdevices are not present.

Add softdep for tegra-drm to make userspace interface available
without module autoloading through device tree entries.

Signed-off-by: Vamsee Vardhan Thummala <vthummala@nvidia.com>
[mperttunen@nvidia.com: some rewording]
Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20250708-host1x-allow-no-subdevs-v1-1-93c66c251f03@nvidia.com