gpu: host1x: Rename "parent" to "host"

gpu: host1x: Rename "parent" to "host"

Rename the host1x clients' parent to "host" because that more closely
describes what it is. The parent can be confused with the parent device
in terms of the device hierarchy. Subsequent patches will add a new
member that refers to the parent in that hierarchy.

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