media: ti-vpe: cal: Move async notifiers from contexts to cal_dev

There's no need to create one async notifier per CAL context. Merge them
all in a single notifier, stored in cal_dev.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>