media: ipu3-imgu: Prefix functions with imgu_* instead of ipu3_*

This addresses the below TODO item, no function related changes:

- Prefix imgu for all public APIs, i.e. change ipu3_v4l2_register() to
  imgu_v4l2_register(). (Sakari)

The changes were obtained by applying the following perl script
to driver code under drivers/staging/media/ipu3/.

perl -pi.back -e 's/ipu3_(?!uapi)/imgu_/g;'

Signed-off-by: Yong Zhi <yong.zhi@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>