[media] vp702x: preallocate memory on device probe

This sets up a buffer and a mutex protecting that buffer in
the struct vp702x_device_state.

The definition of struct vp702x_device_state is moved into the header
in order to use the buffer also in the frontend.

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