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>

drivers/gpu/drm/sti/Makefile

@@ -1,3 +1,6 @@
+sticompositor-y := \
+	sti_gdp.o
+
 stihdmi-y := sti_hdmi.o \
 	sti_hdmi_tx3g0c55phy.o \
 	sti_hdmi_tx3g4c28phy.o \
@@ -7,4 +10,5 @@ obj-$(CONFIG_DRM_STI) = \
 	sti_vtac.o \
 	stihdmi.o \
 	sti_hda.o \
-	sti_tvout.o
+	sti_tvout.o \
+	sticompositor.o