drm: xlnx: zynqmp_dpsub: Configure blender in zynqmp_disp_enable()

To prepare for control of the blender outside of the CRTC code, move the
setup of the blender to the zynqmp_disp_enable() function. This doesn't
introduce any functional change.

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