[media] mem2mem: set missing v4l2_dev pointer

The m2m-deinterlace, mem2mem_testdev and mx2_emmaprp drivers didn't set
the v4l2_dev pointer in struct video_device, even though a v4l2_device was
registered correctly. These days this v4l2_dev pointer must be set correctly,
so this patch adds that for these three drivers.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>