media: mtk-mdp: Add locks for capture and output vb2_queues

Use the mutex in struct mtk_mdp_ctx to protect the
capture and output  vb2_queues. This allows to replace
the ad-hoc wait_{prepare, finish} with
vb2_ops_wait_{prepare, finish}.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>