gpu: host1x: Fix a few sparse warnings

Include the bus.h header, so that various function declarations are
visible in the source file that implements those functions. This keeps
sparse from suggesting that they should be made static.

Make the host1x_bus_type variable static since it isn't used globally.

Finally replace the slightly unsafe dev_set_name(dev, name) by the more
secure dev_set_name(dev, "%s", name).

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