media: stm32-dcmi: revisit buffer list management

Cleanup "active" field usage and enhance list management
to avoid exceptions when releasing buffers on error or
stopping streaming.

Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>