[media] v4l: vsp1: Fix struct vsp1_drm documentation

The struct vsp1_drm references a member 'planes' which doesn't exist.
Correctly identify this documentation against the 'inputs'

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