[media] bttv: fix try_fmt_vid_overlay and setup initial overlay size

try_fmt_vid_overlay should map incorrect sizes and fields to valid values.
It also expects that an initial overlay size is defined so g_fmt_vid_overlay
returns valid information.

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