gpu/host1x: bus: Add missing description for 'driver'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/host1x/bus.c:40: warning: Function parameter or member 'driver' not described in 'host1x_subdev_add'

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-tegra@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201105144517.1826692-2-lee.jones@linaro.org