gpu: host1x: Cleanup and refcounting for syncpoints

Add reference counting for allocated syncpoints to allow keeping
them allocated while jobs are referencing them. Additionally,
clean up various places using syncpoint IDs to use host1x_syncpt
pointers instead.

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