Before this patch, vivi were simulating a scatter gather DMA transfer.
While this is academic, showing how stuff really works on a real PCI 
device, this means a non-optimized code. 
There are only two memory models that vivi implements:
	1) kernel alloced memory. This is also used by read() method.
	   On this case, a vmalloc32 buffer is allocated at kernel;
	2) userspace allocated memory. This is used by most userspace apps.
	   video-buf will store this pointer.
	   a simple copy_to_user is enough to transfer data.
The third memory model scenario supported by video-buf is overlay mode. 
This model is not implemented on vivi and unlikely to be implemented on 
newer drivers, since now, most userspace apps do some post-processing 
(like de-interlacing).
After this patch, some cleanups may be done at video-buf.c to avoid 
allocating pages, when the driver doesn't need a PCI buffer. This is the 
case of vivi and usb drivers.
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>