[media] vivid: support for contiguous DMA buffers

It allows to simulate the behavior of hardware with such limitations or
to connect vivid to real hardware with such limitations.

Add the "allocators" module parameter option to let vivid use the
dma-contig instead of vmalloc.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Vincent Abriou <vincent.abriou@st.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Tested-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>