drm: zte: add overlay plane support

It enables VOU VL (Video Layer) to support overlay plane with scaling
function.  VL0 has some quirks on scaling support.  We choose to skip it
and only adds VL1 and VL2 into DRM core for now.

Function zx_plane_atomic_disable() gets moved around with no changes to
save a forward declaration.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>