[media] dib0700: protect the dib0700 buffer access

This patch protects the common buffer access inside the dib0700 in order
to manage concurrent access. This protection is done using mutex.

Cc: Mauro Carvalho Chehab <mchehab@redhat.com>
Cc: Florian Mickler <florian@mickler.org>
Cc: stable@kernel.org
Signed-off-by: Javier Marcet <javier@marcet.info>
Signed-off-by: Olivier Grenie <olivier.grenie@dibcom.fr>
Signed-off-by: Patrick Boettcher <patrick.boettcher@dibcom.fr>
[mchehab@redhat.com: dprint requires 3 arguments. Replaced by dib_info]
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>