media: staging: rkisp1: remove the 'is_streaming' field from stats and params

The params and stats entities have a field 'is_streaming'.
This field is not needed since the entities can have available
buffers only if they stream and therefore it is enough to
check if there are buffers available.
As a result, their start_stream callbacks can be removed since
they only set the 'is_streaming' field.

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