drm/vc4: Prefer PPF over TPZ when dst >= 2/3 src

The HVS spec recommends using PPF when the downscaling ratio is
between 2/3 and 1. Let's modify vc4_get_scaling_mode() to follow this
recommendation.
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181109102633.32603-2-boris.brezillon@bootlin.com

drivers/gpu/drm/vc4/vc4_plane.c

@@ -129,12 +129,12 @@ static const struct hvs_format *vc4_get_hvs_format(u32 drm_format)
 
 static enum vc4_scaling_mode vc4_get_scaling_mode(u32 src, u32 dst)
 {
-	if (dst > src)
+	if (dst == src)
+		return VC4_SCALING_NONE;
+	if (3 * dst >= 2 * src)
 		return VC4_SCALING_PPF;
-	else if (dst < src)
-		return VC4_SCALING_TPZ;
 	else
-		return VC4_SCALING_NONE;
+		return VC4_SCALING_TPZ;
 }
 
 static bool plane_enabled(struct drm_plane_state *state)