media: videobuf2: check ->synced flag in prepare() and finish()

This simplifies the code a tiny bit and let's us to avoid
unneeded ->prepare()/->finish() calls.

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>