media: vim2m: Remove unneeded buffer lock

This spinlock is used solely to call v4l2_m2m_buf_done().

Since buffers are obtained only after being removed
from the ready queue, there's no concurrent access, and
so no need for synchronization.

Remove the spinlock to make sure no one copies this pattern.

Some archaeology shows this is a small leftover from ancient code.
This driver (then called m2m_testdev) used the videobuf1 framework;
commit d80ee38c ("[media] v4l: mem2mem: port m2m_testdev to vb2")
converted it to videobuf2. The spinlock was then no longer needed,
and this simply went unnoticed.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>