drm/mediatek: Refactor plane init

Add a couple of functions which enumerate the number of planes for a
component and initialize the planes for a component.

No functional changes in this patch, but it will allow us to selectively
support rotation if the component supports it.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>