[media] videobuf: make unused exported functions static

The videobuf_dma_init* and videobuf_dma_map() functions are no longer
used except in videobuf-dma-sg.c itself. Make them static.

These functions were abused in various drivers. All those drivers
have now been fixed, so by no longer exporting these functions
future abuse is now prevented.

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