media: staging/imx: Improve pipeline searching

Export find_pipeline_pad(), renaming to imx_media_pipeline_pad(), and
extend its functionality to allow searching for video devices in the
enabled pipeline in addition to sub-devices.

As part of this:

- Rename imx_media_find_mipi_csi2_channel() to
  imx_media_pipeline_csi2_channel().

- Remove imx_media_find_upstream_pad(), it is redundant now.

- Rename imx_media_find_upstream_subdev() to imx_media_pipeline_subdev()
  with an additional boolean argument for searching upstream or downstream.

- Add imx_media_pipeline_video_device() which is analogous to
  imx_media_pipeline_subdev() but searches for video devices.

- Remove imxmd pointer arg from all of the functions above, it was
  never used in those functions. With that change the i.MX5/6 CSI,
  VDIC, and IC sub-devices no longer require the media_device.

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>