drm: nouveau: Don't use i2c_client->driver

The 'driver' field of the i2c_client struct is redundant and is going to be
removed. Use 'to_i2c_driver(client->dev.driver)' instead to get direct access to
the i2c_driver struct.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Martin Peres <martin.peres@labri.fr>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>