gpu: host1x: Allow syncpoints without associated client

Syncpoints don't need to be associated with any client,
so remove the property, and expose host1x_syncpt_alloc.
This will allow allocating syncpoints without prior knowledge
of the engine that it will be used with.

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