[media] vp702x: use preallocated buffer in the frontend

Note: This change is tested to compile only as I don't have the
hardware.

Signed-off-by: Florian Mickler <florian@mickler.org>
Cc: Patrick Boettcher <pb@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>