media: imx: imx7-mipi-csis: Cleanup and fix subdev pad format handling

The subdev set pad format operation currently misbehaves in multiple ways:

- mipi_csis_try_format() unconditionally stores the format in the device
  state, even for V4L2_SUBDEV_FORMAT_TRY.

- The format is never stored in the pad cfg, but the pad cfg format
  always overwrites the format requested by the user.

- The sink format is not propagated to the source.

Fix all this by reworking the set format operation as follows:

1. For the source pad, turn set() into get() as the source format is not
   modifiable.
2. Validate the requested format and updated the stored format
   accordingly.
3. Return the format actually set.
4. Propagate the format from sink to source.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>