V4L/DVB (13239): saa7146: fix memory leakage in pagetable-handling

In buffer_release() the previously allocated pagetables are not
freed,  which might result in a memory leak in certain application
use-cases, where the frame format is changed from planar format to
non-planar format. The fix explicitely frees the page tables when a
format change is done and when buffer_release() is called.

Signed-off-by: Johann Friedrichs <johann.friedrichs@web.de>
Signed-off-by: Michael Hunold <hunold@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>