gpu/host1x: Remove excess parameter in host1x_subdev_add docs

Fixes the following warning when building docs:
../drivers/gpu/host1x/bus.c:50: warning: Excess function parameter 'driver' description in 'host1x_subdev_add'

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170720174746.29100-4-seanpaul@chromium.org