media: videobuf2-core: attach once if multiple planes share the same dbuf

When multiple planes use the same dma buf, each plane will have its own dma
buf attachment and mapping. It is a waste of IOVA space.

This patch adds a dbuf_duplicated boolean in vb2_plane. If a plane's dbuf
is the same as an existing plane, do not create another attachment and
mapping.

Signed-off-by: Yunke Cao <yunkec@chromium.org>
Acked-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>