media: atomisp: Also track buffers in a list when submitted to the ISP

Instead of using an integer to keep count of how many buffers have
been handed over to the ISP (buffers_in_css) move buffers handed
over to the ISP to a new buffers_in_css list_head so that we can
easily loop over them.

This removes the need for atomisp_flush_video_pipe() to loop over
all buffers and then (ab)use the state to figure out if they
were handed over to the ISP.

Since the buffers are now always on a list when owned by the driver
this also allows the buffer_done path on flush vs normal completion
to be unified (both now need a list_del()) and this common code can
now be factored out into a new atomisp_buffer_done() helper.

This is a preparation patch for moving the driver over to
the videobuf2 framework.

Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>