[media] v4l: vsp1: Make pipeline inputs array index by RPF index

The pipeline inputs array stores pointers to all RPFs contained in the
pipeline. It's currently indexed contiguously by adding RPFs in the
order they are found during graph walk. This can't easily support
dynamic addition and removal of RPFs while streaming, which will be
required for combined VSP+DU support.

Make the array indexed by RPF index instead and skip NULL elements when
iterating over RPFs.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>