To be consistent with the rest of the mem2mem helpers,
rename vb2_m2m_request_queue to v4l2_m2m_request_queue.

This is just a cosmetic change.

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