drm_hwcomposer: Split the drm compositor into per-display threads

drm_hwcomposer: Split the drm compositor into per-display threads

This patch splits out the current single drm compositor with
per-display compositors, each with their own thread.

The per-display compositors are hidden behind a singleton
drm compositor. This allows us to maintain a whole-world view
of all displays involved in a frame. This becomes useful if
we start switching up crtcs/encoders for the displays.
This also allows us to issue one DrmComposition when the
frame is being assembled.

The single DrmComposition handles the plane allocation (since they
might switch between displays), and contains per-display compositions
which are used to store the layer->plane/crtc information for each
frame. The display compositors use the per-display compositions to
display the frame on their output.

Each display compositor receives a shared pointer to the frame's
DrmComposition on QueueComposition. As a result, both the composition,
and the per-display compositions, live for as long as any one
display is still using it. While this is sub-optimal (since a display
might never update again), this is probably fine for now.

Finally, splitting things up per-display will allow us to inject
non-compositing jobs into the composite queue. An example would be
turning the display off, or setting the mode. This ensures that all
frames in the composite queue are displayed before the mode changes
or the display is disabled.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Change-Id: I8a233ea64710b238f70acbcde1f6d771e297b069