drm/mediatek: use layer_nr function to get layer number to init plane

This patch use layer_nr function to get layer number to init plane

When plane init in crtc create,
it use the number of OVL layer to init plane.
That's OVL can read 4 memory address.

For mt2712 third ddp, it use RDMA to read memory.
RDMA can read 1 memory address, so it just init one plane.

For compatibility, this patch use mtk_ddp_comp_layer_nr function
to get layer number from their HW component in ddp for plane init.

Signed-off-by: Stu Hsieh <stu.hsieh@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>