drm: xlnx: zynqmp_dpsub: Don't pass CRTC to zynqmp_disp_setup_clock()

To prepare for usage of the clock setup function outside of the CRTC
code, replace the DRM-specific structures passed as parameters with a
pointer to the zynqmp_disp and the requested clock rate. This doesn't
introduce any functional change.

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