drm/sun4i: rgb: Remove the bridge enable/disable functions

The atomic helpers already call the drm_bridge_enable on our behalf,
there's no need to do it a second time.

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