V4L/DVB: saa7146_vv: fix regression where v4l2_device was registered too late

v4l2_device_register needs to be called before the i2c subdevs are loaded.
However, it was called afterwards in the saa7146 driver. This caused an oops
when loading the mxb and hexium drivers. The vv_init function is now split
into two: one registers the v4l2_device, the other does the rest of the
initialization. The three drivers that depend on this have been updated to
call the new vv_devinit function.

Thanks to Michael Hunold for reporting this.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>