gpu: host1x: Fix host1x driver shutdown

Shutting down a host1x device currently crashes if the device has failed
to probe. The root cause is that the host1x shutdown is implemented as a
struct bus_type callback, but in turn relies on the driver bound to the
device. On failure to probe, no driver will be bound and cause the code
to crash.

Fix this by moving the ->probe(), ->remove() and ->shutdown() callbacks
to the driver rather than the bus.

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