media: videobuf2: split buffer cache_hints initialisation

V4L2 is not the perfect place to manage vb2 buffer cache hints.
It works for V4L2 users, but there are backends that use vb2 core
and don't use V4L2. Factor out buffer cache hints initialisation
and call it when we allocate vb2 buffer.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>