V4L/DVB (10791): videodev: not possible to register NULL video_device

video_register_device_index() checks if it was passed a NULL video_device
pointer (which isn't allowed) _after_ it has already dereferenced it with
video_get_drvdata(vdev).  The checks are clearly pointless and can be
removed, as the function would have crashed before reaching them if vdev
ever was NULL.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>