media: videobuf2-v4l2: add vb2_request_queue/validate helpers

The generic vb2_request_validate helper function checks if
there are buffers in the request and if so, prepares (validates)
all objects in the request.

The generic vb2_request_queue helper function queues all buffer
objects in the validated request.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>