drm: sti: add Compositor

Compositor control all the input sub-device (VID, GDP)
and the mixer(s).
It is the main entry point for composition.
Layer interface is used to control the abstracted layers.

Add debug in mixer and GDP.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Rob Clark <robdclark@gmail.com>