drm: sti: clear all mixer control

Make sure that mixer control register is correctly reset
before use it.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
diff --git a/drivers/gpu/drm/sti/sti_drm_crtc.c b/drivers/gpu/drm/sti/sti_drm_crtc.c
index 36a1ad3..d42c63f 100644
--- a/drivers/gpu/drm/sti/sti_drm_crtc.c
+++ b/drivers/gpu/drm/sti/sti_drm_crtc.c
@@ -38,6 +38,8 @@
 		if (clk_prepare_enable(compo->clk_compo_aux))
 			DRM_INFO("Failed to prepare/enable compo_aux clk\n");
 	}
+
+	sti_mixer_clear_all_layers(mixer);
 }
 
 static void sti_drm_crtc_commit(struct drm_crtc *crtc)