drm: xlnx: zynqmp_dpsub: Move CRTC to zynqmp_dpsub structure

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

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