video: adf: support "simple" buffers

Simple buffers are linear RGB buffers analogous to KMS's dumb buffers.
Simple buffers can be allocated and posted to a display interface
without any driver-private data.

Internally, ADF drivers provide the driver-private data needed (if any)
to post a simple buffer to the display.

Change-Id: Ib0b737622eaf343111310f6623f99d69cf3807d2
Signed-off-by: Greg Hackmann <ghackmann@google.com>