drm/xe/i2c: cancel the client work on remove

In the Linux kernel, the following vulnerability has been resolved:

drm/xe/i2c: cancel the client work on remove

xe_i2c_notifier() stores the DesignWare adapter in i2c->adapter and
schedules i2c->work when the adapter is registered under the xe I2C
platform device, and xe_i2c_client_work() then instantiates the AMC
client device on that adapter.

xe_i2c_remove() tears down the AMC, unregisters the client devices,
the bus notifier and the adapter platform device, but it never drains
i2c->work. A work item that is still queued or running when the
adapter is unregistered dereferences i2c->adapter in
i2c_new_client_device() after platform_device_unregister() has
released the adapter. The work item is also embedded in the
devm-allocated struct xe_i2c, so a work item still queued after the
drm device devm unwind frees that allocation runs its callback on
freed memory.

The bus notifier is the only thing that schedules this work, and it is
unregistered after the client devices. An instance that is still queued
when the teardown runs can therefore write
i2c->client[XE_I2C_CLIENT_AMC] while the loop is unregistering and
clearing the same array, and an AMC client it instantiates late is only
cleaned up by the adapter's own child sweep in i2c_del_adapter().

Move bus_unregister_notifier() in front of the client teardown loop
and cancel the work right after it, so no new instance can be
scheduled and a queued instance is drained before the client array is
touched. A running instance still finds a live adapter, since the
adapter is unregistered later.

(cherry picked from commit a23f27d5ec11c3f418c5e5531eea34ccda2b5b20)