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

The 'driver' field of the i2c_client struct is redundant and is going to be
removed. The results of the expressions 'client->driver.driver->field' and
'client->dev.driver->field' are identical, so replace all occurrences of the
former with the later. To get direct access to the i2c_driver struct use
'to_i2c_driver(client->dev.driver)'.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>