drm_hwcomposer: add scene flattening

Flattening of a scene is triggered if it doesn't change for a while.
As of now there is a separate thread which triggers flattening if the
scene did not change in last 60 vsyncs.

There are two options for flattening a scene:
* Serial, by using a writeback connector attached to the same crtc as
  the one driving the display. This happens only if possible_clones
  mask reports that the display encoder and writeback encoder could
  work simultaneously.
The steps for achieving this are:
1. Build a commit that enables writeback connector, we don't need to
   build a commit that contains the entire active_composition, just
   set the writeback specific properties a let the kernel duplicate
   the rest of the state.
2. Commit and wait for writeback_fence to fire.
3. Setup a composition with just one plane enabled.
4. Apply the composition if a new one has not been applied meanwhile.

* Concurrent, by comitting the scene to a new unused crtc (state !=
  DRM_MODE_CONNECTED) and getting the result back through a writeback
  connector.
The steps for achieving this are:
1. Copy layers from active composition.
2. Plan layers of copy on the unused crtc. This is realized by using a
   newly created DrmDisplayCompositor object.
3. Commit copy to the unsed crtc and get the result as a drmhwclayer.
4. Setup a composition with just one plane enabled. Re-importing the
   buffers might be needed since we might have been using a different
   dri node.
5. Apply the composition if a new one has not been applied while doing
   the flattening

Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
1 job for scene_flattening in 40 seconds (queued for 1 second)
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