[media] media: davinci: vpif_display: drop buf_cleanup() callback

this patch drops buf_cleanup() callback as this callback
is never called with buffer state active.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>