drm/sti: use vtg array instead of vtg_main/aux

This is more generic and more consistent with the other members of the
sti_compositor struct.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Acked-by: Vincent Abriou <vincent.abriou@st.com>
diff --git a/drivers/gpu/drm/sti/sti_gdp.c b/drivers/gpu/drm/sti/sti_gdp.c
index 083c0ac..5ba470b 100644
--- a/drivers/gpu/drm/sti/sti_gdp.c
+++ b/drivers/gpu/drm/sti/sti_gdp.c
@@ -650,8 +650,7 @@
 
 	if (!gdp->vtg) {
 		/* Register gdp callback */
-		gdp->vtg = mixer->id == STI_MIXER_MAIN ?
-					compo->vtg_main : compo->vtg_aux;
+		gdp->vtg = compo->vtg[mixer->id];
 		if (sti_vtg_register_client(gdp->vtg,
 					    &gdp->vtg_field_nb, crtc)) {
 			DRM_ERROR("Cannot register VTG notifier\n");