gpu: host1x: Request channels for clients, not devices

gpu: host1x: Request channels for clients, not devices

A struct device doesn't carry much information that a channel might be
interested in, but the client very much does. Request channels for the
clients rather than their parent devices and store a pointer to them
in order to have that information available when needed.

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