media: staging: rkisp1: cap: protect access to buf with the spin lock

The function 'rkisp1_stream_start' calls 'rkisp1_set_next_buf'
which access the buffers, so the call should be protected by
taking the cap->buf.lock.
After this patch, all locks are reviewed and commented so remove
the TODO item "review and comment every lock"

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>