media: omap_vout: convert to vb2

Convert omap_vout to the vb2 framework. After this change
'v4l2-compliance -s' passes all tests.

As usual, this vb2 conversion is a 'big bang' patch and hard to read.

Tested on a Pandaboard and a Beagle XM board.

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