gpu: host1x: Correct trivial kernel-doc inconsistencies

Silence documentation build warnings by adding kernel-doc fields.

./include/linux/host1x.h:69: warning: Function parameter or member 'parent' not described in 'host1x_client'
./include/linux/host1x.h:69: warning: Function parameter or member 'usecount' not described in 'host1x_client'
./include/linux/host1x.h:69: warning: Function parameter or member 'lock' not described in 'host1x_client'

Signed-off-by: Colton Lewis <colton.w.lewis@protonmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>