media: ipu3-imgu: Refactor bytesperpixel calculation

Move the calculation to an inline function, to it can be used by other
parts of the driver.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>