gpu: host1x: syncpt: Request syncpoints per client

Rather than request syncpoints for a struct device *, request them for a
struct host1x_client *. This is important because subsequent patches are
going to break the assumption that host1x will always be the parent for
devices requesting a syncpoint. It's also a more natural choice because
host1x clients are really the only ones that will know how to deal with
syncpoints.

Note that host1x clients are always guaranteed to be children of host1x,
regardless of their location in the device tree.

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