This is step one of supporting horizontal and vertical downsampling.
This just adds support for the h/vdownsampling fields and it increases
the maximum number of planes to 3.

Currently none of the planar formats need horizontal or vertical
downsampling, so this change has no effect at the moment.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>