media: ipu3-cio2: Introduce to_cio2_buffer() helper macro

Provide to_cio2_buffer() helper macro and convert users to make
code easier to read.

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>