[media] saa7134: convert to vb2

Convert the saa7134 driver to vb2.

Note that while this uses the vb2-dma-sg version, the VB2_USERPTR mode is
disabled. The DMA hardware only supports DMAing full pages, and in the
USERPTR memory model the first and last scatter-gather buffer is almost
never a full page.

In practice this means that we can't use the VB2_USERPTR mode.

This has been tested with raw video, compressed video, VBI, radio, DVB and
video overlays.

Unfortunately, a vb2 conversion is one of those things you cannot split
up in smaller patches, it's all or nothing. This patch switches the whole
driver over to vb2, using the vb2 ioctl and fop helper functions.

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