drm/meson: move OSD scaler management into plane atomic update

In preparation to support the Primary Plane scaling, move the basic
OSD Interlace-Only scaler setup code into the primary plane atomic
update callback and handle the vsync scaler update like the overlay
plane scaling registers update.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1541497202-20570-3-git-send-email-narmstrong@baylibre.com