V4L/DVB (7566): videobuf-dvb: allow its usage with videobuf-vmalloc

videobuf-dvb were still using a function that were videobuf-dma-sg
dependent. This patch creates a generic handler for this function. This
way, videobuf-dvb can now work with all videobuf implementations.

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