drivers: media: cfe: Find the source pads on the sensor entity

The driver was assuming that pad 0 on the sensor entity was the
appropriate source pad, but this isn't necessarily the case.
With video-mux, it has the sink pads first, and then the source
pad as the last one.

Iterate through the sensor pads to find the relevant source pads.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>