media: vimc: Remove or modify stream checks

Change the way subdevices check if the stream is running. Verify the stream
pointer instead of src_frame. This makes easier to get rid of the void* and
u8* that points to frames in the subdevices structs.

Remove checks that s_stream does on subdevices. They are redundant since
the Media Controller Framework doesn't allow two streaming on the same
media pipeline at the same time.

Signed-off-by: André Almeida <andrealmeid@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+samsung@kernel.org>