drm: sti: add GDP layer

Generic Display Pipeline are one of the compositor input sub-devices.
GDP are dedicated to graphic input like RGB plans.
GDP is part of Compositor hardware block which will be introduce later.

A sti_layer structure is used to abstract GDP calls from Compositor.

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

diff --cc drivers/gpu/drm/sti/Makefile
Simple merge

diff --cc drivers/gpu/drm/sti/sti_gdp.c
Simple merge

diff --cc drivers/gpu/drm/sti/sti_gdp.h
Simple merge

diff --cc drivers/gpu/drm/sti/sti_layer.h
Simple merge