[media] media: blackfin: use vb2_ops_wait_prepare/finish helper

This patch drops driver specific wait_prepare() and
wait_finish() callbacks from vb2_ops and instead uses
the the helpers vb2_ops_wait_prepare/finish() provided
by the vb2 core, the lock member of the queue needs
to be initalized to a mutex so that vb2 helpers
vb2_ops_wait_prepare/finish() can make use of it.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Acked-by: Scott Jiang <scott.jiang.linux@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>