media: videobuf2: move cache_hints handling to allocators

This moves cache hints handling from the videobuf2 core down
to the allocator's level, because allocators do the sync/flush
caches eventually and may take better decisions. Besides,
allocators already decide whether cache sync/flush should
be done or can be skipped. This patch moves the scattered
buffer cache sync logic to one common place.

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>