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