[media] media: au0828 audio mixer isn't connected to decoder

When snd_usb_audio gets probed first, audio mixer doesn't get linked to
the decoder.

Change au0828_media_graph_notify() to handle the mixer entity getting
registered before the decoder.

Change au0828_media_device_register() to invoke
au0828_media_graph_notify() to connect entites that were created prior
to registering the notify handler.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Reported-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>