drm/sun4i: rgb: Enable panel after controller

The panel should be enabled after the controller so that we do not have
visual glitches on the panel while the controller is setup. Similarly,
the panel should be disabled before the controller.

Signed-off-by: Jonathan Liu <net147@gmail.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>