media: vim2m: initialize the media device earlier

Before the video device node is registered, the v4l2_dev.mdev
pointer must be set in order to correctly associate the video
device with the media device. Move the initialization of the
media device up.

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