[media] v4l2-ioctl: set readbuffers to 2 in g_parm

If g_parm is handled automatically, then set readbuffers to 2, which is the
minimum number of buffers videobuf uses.

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