libcamera: pipeline: simple: Support camera sensors that contain an ISP

Camera sensors can include an ISP. For instance, the AP1302 external ISP
can be connected to up to two raw camera sensors, and the combination of
the sensors and ISP is considered as a (smart) camera sensor from
libcamera's point of view.

The CameraSensor class has limited support for this already. Extend the
simple pipeline handler to support such sensors, by using the media
entity corresponding to the ISP instead of the raw camera sensor's
entity.

We don't need to handle the case where an entity in the SoC would expose
the MEDIA_ENT_F_PROC_VIDEO_ISP function, as pipeline containing an ISP
would have a dedicated pipeline handler.

The implementation is limited as it won't support other multi-entity
camera sensors (such as CCS). While this would be worth supporting, we
don't have a test platform with a CCS-compatible sensor at this point,
so let's not over-engineer the solution. Extending support to CCS (and
possibly other sensor topologies) will likely involve helpers that can
be used by other pipeline handlers (such as generic graph walk helpers
for instance) and extensions to the CameraSensor class.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

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