drm: xlnx: zynqmp_dpsub: Move planes to zynqmp_dpsub structure

Decouple the zynqmp_disp, which handles the hardware configuration, from
the DRM planes by moving the planes to the zynqmp_dpsub structure. The
planes handling code will be moved to a separate file in a subsequent
step.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>