media: videobuf2-core: add request helper functions

Add a new helper function to tell if a request object is a buffer.

Add a new helper function that returns true if a media_request
contains at least one buffer.

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>