[media] v4l: vsp1: wpf: Clear RPF to WPF association at stream off time

The VSP1 stores the video pipelines' input (RPF) to output (WPF)
mappings in a WPF register. An RPF must never be associated with
multiple WPFs, even if all of those WPFs but one are unused, otherwise
the hardware won't function properly.

The driver doesn't ensure this correctly as it never clears the
mappings. An RPF used with one WPF and later with a different WPF will
lead to malfunction, as it will be associated with two WPFs. Clear the
mappings at stream off time to fix this.

Reported-by: Damian Hobson-Garcia <dhobsong@igel.co.jp>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>