media: v4l: vsp1: Use vsp1_entity.pipe to check if entity belongs to a pipeline

The DRM pipeline handling code uses the entity's pipe list head to check
whether the entity is already included in a pipeline. This method is a
bit fragile in the sense that it uses list_empty() on a list_head that
is a list member. Replace it by a simpler check for the entity pipe
pointer.

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