drm/bridge: analogix-anx78xx: convert to i2c_new_dummy_device

The i2c_new_dummy() function is deprecated since it returns NULL on
error. Change this to use the recommended replacement
i2c_new_dummy_device() that returns an error code that can be read with
PTR_ERR() and friends.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190815004854.19860-5-masneyb@onstation.org