media: ipu3-cio2: Switch to use media_entity_to_video_device()

V4L2 provides a few helper macros, in particular
media_entity_to_video_device(). Switch the driver
to use it instead of open-coded variant.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>