gpu: host1x: Fix memory leak of device names

The device names allocated by dev_set_name() need be freed
before module unloading, but they can not be freed because
the kobject's refcount which was set in device_initialize()
has not be decreased to 0.

As comment of device_add() says, if it fails, use only
put_device() drop the refcount, then the name will be
freed in kobejct_cleanup().

device_del() and put_device() can be replaced with
device_unregister(), so call it to unregister the added
successfully devices, and just call put_device() to the
not added device.

Add a release() function to device to avoid null release()
function WARNING in device_release(), it's empty, because
the context devices are freed together in
host1x_memory_context_list_free().

Fixes: 8aa5bcb6 ("gpu: host1x: Add context device management code")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>