media: saa7146: drop overlay support

Destructive overlay support (i.e. where the video frame is DMA-ed
straight into a framebuffer) is effectively dead. It was a
necessary evil in the early days when computers were not fast enough
to copy SDTV video frames around, but today that's no longer a problem.

It requires access to the framebuffer memory, which is a bad idea and
very hard to do safely. In addition, in drm it is today almost
impossible to get hold of the framebuffer address.

So drop support for this.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>