V4L/DVB (6255): Convert vivi to use videobuf-vmalloc

This patch removes the usage of videobuf-dma-sg from vivi driver, using
instead videobuf-vmalloc. This way, vivi will be useful for testing the
newer method. Reverting this patch won't hurt vivi, since both methods
work fine.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>