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>

media: staging/intel-ipu3: css: Add dma buff pool utility functions

The pools are used to store previous parameters set by
user with the parameter queue. Due to pipelining,
there needs to be multiple sets (up to four)
of parameters which are queued in a host-to-sp queue.

[mchehab@kernel.org: fixed two minor issues on comments: a space before
tab and a typo: "vaid" -> "valid"]
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>