gpu: host1x: Make host1x_client_unregister() return void

This function returned zero unconditionally. Make it return no value and
simplify all callers accordingly.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Thierry Reding <treding@nvidia.com>