media: videobuf2-v4l2.c: add vb2_queue_change_type() helper

media: videobuf2-v4l2.c: add vb2_queue_change_type() helper

On some platforms a video device can capture either video data or
metadata. The driver can implement vidioc functions for both video and
metadata, and use a single vb2_queue for the buffers. However, vb2_queue
requires choosing a single buffer type, which conflicts with the idea of
capturing either video or metadata.

The buffer type of vb2_queue can be changed, but it's not obvious how
this should be done in the drivers. To help this, add a new helper
function vb2_queue_change_type() which ensures the correct checks and
documents how it can be used.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>