[media] videobuf-dma-contig: remove support for cached mem

videobuf_queue_dma_contig_init_cached() is not used anywhere.
Drop support for it, cleaning up the code a little bit.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>