media: videobuf2: don't test db_attach in dma-contig prepare and finish

We moved cache management decision making to the upper layer and
rely on buffer's need_cache_sync flags and videobuf2 core. If the
upper layer (core) has decided to invoke ->prepare() or ->finish()
then we must sync.

For DMABUF ->need_cache_sync_on_prepare and ->need_cache_sync_on_flush
are always false so videobuf core does not call ->prepare() and
->finish() on such buffers.

Additionally, scratch the DMABUF comment.

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>