videonode: imx6: cleanup pixel formats

All formats are now tested.

Also, don't use the alpha channel. This is not quite correct for formats
with alpha channel but for video probably a good idea.

Change-Id: I4a2404d7c96023bb1e3fca41e1f224d23347f290
Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>