media: videobuf2: do not sync caches when we are allowed not to

Skip ->prepare() or/and ->finish() cache synchronisation if
user-space requested us to do so (or when queue dma direction
permits us to skip cache syncs).

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>