[media] videobuf-dma-contig: add cache support

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Acked-by: Giancarlo Asnaghi <giancarlo.asnaghi@st.com>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>