[media] bdisp: composing support

Support the composing (at VIDEO_CAPTURE) with the _selection API.
v4l2-compliance successfully run ("test Composing: OK")

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>