This supports 420 and 422 subsampling with 2 or 3 planes, tested with
modetest.  It doesn't set up chroma subsampling position (which it
appears KMS doesn't deal with yet).

The LBM memory is overallocated in many cases, but apparently the docs
aren't quite correct and I'll probably need to look at the hardware
source to really figure it out.
Signed-off-by: Eric Anholt <eric@anholt.net>