drm: xlnx: zynqmp_dpsub: Pass disp structure to all internal functions

The internal functions dealing with the audio/video buffer manager, the
blender, and the audio mixer, all receive pointers to the respective
objects. Those objects are embedded in the zynqmp_disp structure, and a
very small. Treating them as separate objects would require expanding
them with back-pointers to the zynqmp_disp in order to access fields
such as the device pointer for debug messages, and this isn't worth it.

Instead, merge those structures with the zynqmp_disp structure, and pass
the zynqmp_disp pointer to all internal functions.

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