[media] media: davinci: vpif_capture: improve vpif_buffer_prepare() callback

this patch improve vpif_buffer_prepare() callback, as buf_prepare()
callback is never called with invalid state and check for
vb2_plane_vaddr(vb, 0) is dropped as payload check should
be done unconditionally.

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>