RFC: media: Add media_request_{pin,unpin} API

This is probably not the API we will want to add, but it
should show what semantics are needed by drivers.

The goal is to allow the OUTPUT (aka source) buffer and the
controls associated to a request to be released from the request,
and in particular return the OUTPUT buffer back to userspace,
without signalling the media request fd.

This is useful for devices that are able to pre-process
the OUTPUT buffer, therefore able to release it before
the decoding is finished. These drivers should signal
the media request fd only after the CAPTURE buffer is done.

Tested-by: John Cox <jc@kynesim.co.uk>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>