drm: sti: add Mixer

Mixer hardware IP is responsible of mixing the different inputs layers.
Z-order is managed by the mixer.
We could 2 mixers: one for main path and one for auxillary path

Mixers are part of Compositor hardware block

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Rob Clark <robdclark@gmail.com>
diff --git a/drivers/gpu/drm/sti/Makefile b/drivers/gpu/drm/sti/Makefile
index 9b613d2..4a3a816 100644
--- a/drivers/gpu/drm/sti/Makefile
+++ b/drivers/gpu/drm/sti/Makefile
@@ -1,4 +1,5 @@
 sticompositor-y := \
+	sti_mixer.o \
 	sti_gdp.o \
 	sti_vid.o
 
@@ -12,4 +13,4 @@
 	stihdmi.o \
 	sti_hda.o \
 	sti_tvout.o \
-	sticompositor.o
+	sticompositor.o
\ No newline at end of file