v4l: vsp1: Start and stop DRM pipeline independently of planes

The KMS API supports enabling a CRTC without any plane. To enable that
use case, we need to start the pipeline when configuring the LIF,
instead of when enabling the first plane.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Acked-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>