media: vb2: Introduce a vb2_get_buffer accessor

Some drivers need to access a vb2 buffer from its
queue index. Introduce an accessor to abstract this,
and avoid drivers from accessing private members.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>