[media] cx88: remove leftover start_video_dma() call

The start_streaming op is responsible for starting the video dma,
so it shouldn't be called anymore from the buf_queue op.

Unfortunately, this call to start_video_dma() was added to the
start_streaming op, but was forgotten to be removed from the
buf_queue op, which is where it used to be before the vb2 conversion.

Calling this function twice causes very hard to find errors: sometimes
it works, sometimes it doesn't. It took me a whole friggin' day
to track this down, and in the end it was just luck that my eye suddenly
triggered on that line.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>