[media] media: Check for active links on pads with MEDIA_PAD_FL_MUST_CONNECT flag

Do not allow streaming if a pad with MEDIA_PAD_FL_MUST_CONNECT flag is not
connected by an active link.
This patch makes it possible to avoid drivers having to check for the most
common case of link state validation: a sink pad that must be connected.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Tested-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>