media: nuvoton: npcm-video: Fix sleeping in atomic context

Change to use mutex instead of spinlock for buffer list lock to fix
sleeping in atomic context warnings reported by the Smatch tool.

Fixes: 70721089985c ("media: nuvoton: Add driver for NPCM video capture and encoding engine")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Marvin Lin <milkfafa@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>