media: coda: set min_buffers_needed

The current driver implementation expects at least one buffer on
all queues to start streaming. Properly signal this to the vb2
core, to avoid confusion when streamon is racing with qbuf.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>