media: vim2m: Register video device after setting up internals

Prevent NULL (or close to NULL) pointer dereference in various places by
registering the video device only when the V4L2 m2m framework has been set
up.

Fixes: commit 96d8eab5 ("V4L/DVB: [v5,2/2] v4l: Add a mem-to-mem videobuf framework test device")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>